A holiday house or vacation home; a villa used for leisure or seasonal stays.
夏は家族で別荘に行って自然の中で過ごす。
In summer the family goes to the vacation home and spends time in nature.
彼は山の別荘で週末を過ごすのが好きだ。
He likes to spend weekends at his mountain villa.
別荘を貸し切って友人たちとバーベキューをした。
We rented a vacation home and had a barbecue with friends.
Colloquial/slang: prison or jail — a euphemistic way to refer to incarceration (e.g., jokingly saying someone 'went to the villa').
彼は裏金の件で別荘行きになったと聞いた（俗語的表現）。
I heard he went to prison over the bribery case (colloquial expression).
そんな冗談を言うと『別荘』に行くことになるよ、と友達にからかわれた。
I was teased by friends who said that if I joked like that I'd end up 'in the villa' (in jail).