I don't know if the definition is really that grey.
Tom Mitchell's popular definition (from his book "Machine Learning"), which is is even quoted in the opening of the Wikipedia article on Machine Learning, comes to mind:
"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" [1]
[1]: Mitchell, T. (1997). Machine Learning, McGraw Hill. ISBN 0-07-042807-7, p.2.
Tom Mitchell's popular definition (from his book "Machine Learning"), which is is even quoted in the opening of the Wikipedia article on Machine Learning, comes to mind:
"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" [1]
[1]: Mitchell, T. (1997). Machine Learning, McGraw Hill. ISBN 0-07-042807-7, p.2.
[2]: https://en.wikipedia.org/wiki/Machine_learning#Overview