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US /ˈdiː.kriːs/ 
UK /dɪˈkriːs/ 

Oxford Essential Dictionary


 verb (decreases, decreasing, decreased )
to become or to make something smaller or less:
The number of people in the village has decreased from 200 to 100.
 opposite increase

>> decrease noun:
There was a decrease in the number of people living in the village.
 opposite increase

Longman Dictionary of Contemporary English


I. decrease1 /dɪˈkriːs/ BrE AmE verb [intransitive and transitive]
[Date: 1300-1400; Language: Anglo-French; Origin: decreistre, from Latin decrescere, from crescere 'to grow']
to become less or go down to a lower level, or to make something do this ⇨ reduce OPP increase:
The number of people who have the disease has decreased significantly in recent years.
They want to decrease their reliance on oil.
decrease by
Average house prices decreased by 13% last year.
decrease to
By 1881, the population of Ireland had decreased to 5.2 million.
decrease from
The North’s share of the world’s energy consumption is expected to decrease from 70% to 60%.
decrease in
Attacks of asthma decrease in frequency through early adult life.
• • •
■ adverbs
significantly (=enough to be an important change) Violent crime has significantly decreased over the last ten years.
considerably/substantially (=a lot) During this period unemployment decreased considerably.
markedly (=very noticeably) Aircraft collisions decreased markedly during the 1990s.
rapidly (=very quickly) Since then, elephant numbers have been decreasing rapidly.
dramatically (=suddenly and a lot) The survival rate decreases dramatically as the disease progresses.
slightly (=a little) Population levels actually began to slightly decrease five years ago.
steadily (=gradually and continuously) The proportion of adult cigarette smokers has been steadily decreasing.
• • •
decrease to become less in number or amount: The average rainfall has decreased by around 30 percent.
go down to decrease. Go down is less formal than decrease and is the usual word to use in conversation: Unemployment has gone down in the past few months.
decline formal to decrease – used with numbers or amounts, or about the level or standard of something: The standard of living has declined. | Support for the government is steadily declining. | Salaries have declined by around 4.5%.
diminish to become smaller or less important: Union membership diminished from 30,000 at its height to just 2,000 today.
fall/drop to decrease, especially by a large amount. Fall and drop are less formal than decrease: The number of tigers in the wild has fallen to just over 10,000. | At night, the temperature drops to minus 20 degrees.
plunge /plʌndʒ//plummet /ˈplʌmət, ˈplʌmɪt/ to suddenly decrease very quickly and by a very large amount: Share prices have plummeted 29% in the last four months. | Climate change could cause global temperatures to plummet.
slide if a price or value slides, it gradually decreases in a way that causes problems – used especially in news reports: The dollar fell in late trading in New York yesterday and slid further this morning.
dwindle /ˈdwɪndl/ to gradually decrease until there is very little left of something, especially numbers or amounts, popularity, or importance: Support for the theory is dwindling.
taper off /ˈteɪpə $ -ər/ if a number or the amount of an activity that is happening tapers off, it gradually decreases, especially so that it stops completely: Political violence tapered off after the elections.

Oxford Advanced Learner's Dictionary




de·crease [decrease decreases decreased decreasing] verb, noun


verb   [dɪˈkriːs]  ;   [dɪˈkriːs]  intransitive, transitive (rather formal)
to become or make sth become smaller in size, number, etc
~ (from sth) (to sth) The number of new students decreased from 210 to 160 this year.
a decreasing population
Donations have decreased significantly over the past few years.
~ by sth The price of wheat has decreased by 5%.
~ in sth This species of bird is decreasing in numbers every year.
~ sth People should decrease the amount of fat they eat.
Opp:  increase
Verb forms:

Word Origin:
late Middle English: from Old French decreis (noun), decreistre (verb), based on Latin decrescere, from de- ‘down’ + crescere ‘grow’.  
decrease verb
1. I (formal)
The number of students has decreased this year.
falldropdeclinecome downdiminish|especially business sinkslumpplungeplummettumble
Opp: increase
decrease/fall/drop/decline/come down/diminish/sink/slump/plunge/plummet/tumble by 100, 25%, a half, etc.
decrease/fall/drop/decline/come down/diminish/sink/sink/slump/plunge/plummet/tumble from 1 500 to 1 000
decrease/decline/diminish with age, time, experience, etc.
prices decrease/fall/drop/decline/come down/sink/slump/plunge/plummet/tumble
decrease/fall/drop/decline/slump/plunge dramatically
2. T (formal)
People should decrease the amount of sugar in their diets.
reducecutlowerbring sth downcut sth back/cut back on sthcut sth down/cut down on sthminimizeturn sth down|especially AmE business scale sth back|journalism slash
Opp: increase
decrease/reduce/cut/lower/bring down/cut back/cut down/scale back sth from 100 to 75
decrease/reduce/cut/lower/bring down/cut back/cut down/scale back sth by half, 50, etc.
decrease/reduce/cut/lower/bring down/minimize the risk/rate of sth
Decrease or reduce? Decrease is more formal and less frequent than its opposite, increase; it is used especially in the contexts of health and medicine. The more usual opposite of increase is reduce.  
Example Bank:
Average family size has decreased from five to three children.
Crime has decreased by 20 per cent.
Spending has decreased slightly this year.
The heart gradually decreases in size.
• The number of quarrels among children decreases with age.

• The dose was gradually decreased after eight weeks.

Cambridge Advanced Learner's Dictionary

Cambridge Advanced Learner's Dictionary - 4th Edition

decrease / dɪˈkriːs /   / ˈdiː.kriːs / verb [ I or T ]

B1 to become less, or to make something become less:

Our share of the market has decreased sharply this year.

We have decreased our involvement in children's books.

→  Opposite increase

© Cambridge University Press 2013

Collins Advanced Learner’s English Dictionary


 decreases, decreasing, decreased
 (The verb is pronounced [dɪkri͟ːs]. The noun is pronounced [di͟ːkriːs].)
 1) V-ERG When something decreases or when you decrease it, it becomes less in quantity, size, or intensity.
  [V by amount] Population growth is decreasing by 1.4% each year...
  [V from/to amount] The number of independent firms decreased from 198 to 96...
  [V amount] Raw-steel production by the nation's mills decreased 2.1% last week...
  [V in n] Since 1945 air forces have decreased in size...
  [V n] Gradually decrease the amount of vitamin C you are taking when you begin to feel better...
  [V-ing] We've got stable labor, decreasing interest rates, low oil prices.
 2) N-COUNT: oft N in/of n A decrease is a reduction in the quantity, size, or intensity of something.
  In Spain and Portugal there has been a decrease in the number of young people out of work...
  Bank base rates have fallen from 10 per cent to 6 per cent - a decrease of 40 per cent.

Merriam-Webster's Advanced Learner's Dictionary

1de·crease /dɪˈkriːs/ verb -creas·es; -creased; -creas·ing
1 [no obj] : to become smaller in size, amount, number, etc.
• Sales decreased by five percent this year.
• The population is decreasing steadily. = The population is steadily decreasing in size.
2 [+ obj] : to make (something) smaller in size, amount, number, etc.
• By exercising often, you can decrease [=reduce, lower] your chance of developing heart disease.
• The driver decreased her speed as she approached the curve.
• These changes will decrease our expenses.
- opposite increase
- decreased adj
• The patient has a decreased appetite.
• a decreased risk of heart disease
- decreasingly adv
• The medication has been decreasingly effective. [=has gradually been becoming less effective]