The Scandinavian Defense, also called the Center Counter, starts 1.e4 d5. Black attacks e4 immediately. After 2.exd5 Black usually recaptures with the queen, 2...Qxd5, and accepts that White will gain a tempo with 3.Nc3.
Bringing the queen out early breaks a basic opening rule, but here it works: the queen has safe squares on a5, d6 and d8, and Black's pawn structure is very healthy. Black's light-squared bishop also gets out to f5 or g4 before ...e6, as in the Caro-Kann.
The other main choice is 2...Nf6, delaying the recapture so that the knight can take on d5 later. It leads to sharper play, including the Portuguese Variation (3.d4 Bg4) and the Icelandic Gambit (3.c4 e6).