When you see people from your past, you start acting like you did when you used to spend time with them. (i.e. you start acting like a 14-year-old when you unexpectedly run into your freshman year boyfriend at the grocery store)
As made popular by the character of Marshall Erikson on an April 2008 episode of CBS sitcom "How I Met Your Mother".
When Linda, the 50-year old elegant socialite, would squeal with delight, bounce on her heels and speak in 70's youth-vernacular whenever she got together with her sorority sister Patty. Her husband explained to his friend, "It's just Revertigo."