Perimenopause means "around menopause." It's the years-long transition, usually beginning in a woman's mid-40s, when the ovaries gradually wind down and estrogen swings high and low rather than staying steady.
The hallmark sign is changing periods, but it's also when symptoms like hot flashes, sleep problems, and mood changes typically start — see the full list of perimenopause symptoms. It ends at menopause itself, defined as 12 months with no period.
For how it differs from menopause and postmenopause, see perimenopause vs. menopause.