The reference is Shafi Goldwasser, Silvio Micali, Ronald L. Rivest, A Digital Signature Scheme Secure Against Adaptive Chosen-Message Attacks, in SIAM Journal on Computing, 1988.
Selective forgery. Forge a signature for a particular message chosen a priori by the enemy.
Here, "a priori" means, informally, that the message is to some degree chosen by the adversary (or at least restricted) before the attack is carried, as opposed to being entirely a result of the attack.
The authors do not give a formal definition. One could be that the adversary must define, before being given the public key, a suitably small subset of the full message set among which s/he will later be able to exhibit a forgery (that is, an acceptable message/signature pair for which s/he has not obtained the signature by asking a signing oracle).