According to the Urban Dictionary:

“A one night stand, or casual sex. Popularized by Craigslist and dating sites.
I looked up Craigslist to arrange a casual encounter for the weekend.
#one night stand#swns#affair#sex#booty#no strings attached
by Lemming2 July 07, 2009”

One of  MacMillan Dictionary’s definition of ‘encounter’:

“an occasion when people have sex, especially in a way that was not planned”

Huffpost article of 3/23/18 entitled “A Eulogy For Craigslist’s ‘Casual Encounters,’ And All The Sex I Found There” by Emily McCombs

SFWeekly article dated 6/6/18:

“Before Tinder or Grindr, before there was ever Match or, there was the magical yet shameful sexual underground of Craigslist personals. Back in the day before hook-up apps, we’d use Craigslist Casual Encounters for anonymous, tawdry, one-night stands wherein you never called the partner back afterward and you wouldn’t even make eye contact if you saw them on the street.”