Machine learning
Also called: ML
In one sentence
An approach where a system derives its own rules from examples instead of being given rules by a person.
In more depth
In traditional software a developer encodes the logic directly. In machine learning the logic is learned: the system is shown many labelled examples and adjusts internal values until its outputs match the expected ones.
This is why machine learning succeeds at tasks nobody can write rules for — recognising a face, judging whether a review is positive — and why its behaviour depends so heavily on the data it was shown.