Machine learning definitions by different authors

Arthur Samuel  -  Machine learning is a field of study that gives computers the ability to learn without being explicitly programmed.

Explanation: With machine learning we can write a program that will learn things on its own, so with one program for example it can learn to detect if a there is a cat in the picture. Now we don't have to tell the program if each picture has a cat in it or not, the program will learn that itself using machine learning (no explicit programming required).

Tom Mitchell  -  A computer program is said to learn from experience  E with respect to some task  T and some performance measure by P, improves with experience E.

Explanation:  

down vote

Machine learning is a term that is taken from the real world  problem of a person, and applied on something that can't actually learn - a machine.

- machine learning will not (usually) change the code, but it might change it's execution path and decision based on previous data or new gathered data and hence the "learning" effect.

- There are many ways to "teach" a machine - you give weights to many parameter of an algorithm, and then have the machine solve it for many cases, each time you give her a feedback about the answer and the machine adjusts the weights according to how close the machine answer was to your answer or according to the score you gave it's answer, or according to some results test algorithm.