Plain English Meaning
What you do when you can't or shouldn't continue to where you were going, so you change plans and head to a different airport instead. It covers the whole process: deciding to go, picking the new airport, getting there, telling ATC, and landing.
Full Definition
The actions a pilot takes to abandon the intended destination and proceed to an alternate airport when continued flight to the original destination is no longer practical or safe. These procedures include deciding to divert, selecting a suitable alternate, navigating to it, communicating the change with ATC, and arranging the approach and landing under current conditions.
Context Anchor
You see this term in instrument training when , , equipment trouble, instructions, or another makes the original or destination unsuitable.