noun
party clothes; evening wear; formal social attire
Refers to clothing suitable for formal social occasions such as parties, receptions, or evening events. Not everyday wear.
パーティーには社交服を着ていくつもりです。
I plan to wear party clothes to the party.
あの店では男性用の社交服を扱っています。
That shop carries men's evening wear.
ドレス is a loanword for a dress, often implying a one-piece garment for women, while 社交服 is a broader term for social attire for any gender.
Compound of 社交 (しゃこう, 'social') and 服 (ふく, 'clothing'). The term is straightforward and modern, with no obscure historical derivation.