机器学习的定义（尚未有统一的定义），较好的定义如下：
Arthur Samuel(1959).Machine Learning: Field of study that gives computers the ability to learn without being explicitly programmed
在没有明确设置的情况下，使计算机具有学习能力的研究领域
Tom Mitchell(1998). Well-posed Learning Problem: A computer program is said to learn from experience E with respect to some task T and some performance measure P, if its performance on T, as measure by P, improves with experience E.
一个适当的学习问题定义如下：计算机程序从经验E中学习，解决某一任务T，进行某一性能度量P，通过P测定在T上的表现因经验E而提高