Arthur samuel(1959):Machine learning is a field of computer science that gives computers the ability to learn without being explicitly programmed.
Tom mithell(1998):A computer program is said to learn from experience E with respect to some class of tasks T and performance measure P if its performance at tasks in T, as measured by P, improves with experience E.(吴恩达在coursea上曾吐槽他觉得这个定义仅仅是为了押韵)