Money your employer adds when you contribute.
A match is the share of your own contribution your employer pays in alongside it — commonly 50% or 100% of what you put in, up to a percentage of salary.
It is an immediate return on the contributed amount, before any investment return at all. Contributing less than the full match is the only place in personal finance where money is left on the table by default.
Matched money is often subject to a vesting schedule, so it becomes fully yours after a set number of years.