Merriam-Webster's Advanced Learner's Dictionary:
motivation
mo·ti·va·tion /ˌmoʊtəˈveɪʃən/ noun, pl -tions
1 [noncount]
a : the act or process of giving someone a reason for doing something : the act or process of motivating someone
• Some students need motivation to help them through school.
b : the condition of being eager to act or work : the condition of being motivated
• employees who lack motivation
2 : a force or influence that causes someone to do something
[noncount]
• His behavior seemed to be without motivation. = There seemed to be no motivation [=reason, motive] for his behavior.
[count]
• Many people have questioned her motivations in choosing to run for office at this time.
- mo·ti·va·tion·al /ˌmoʊtəˈveɪʃənl̩/ adj
• a motivational speech [=a speech that is intended to motivate people]