A Seme is the dominant partner of the Uke in a Yaoi relationship. The word comes from the verb "to attack."
If the character who has been made the seme is not in a canon that originally has yaoi, it is a very good chance that the character is Out of Character. It should not be used if the characters in question do not usually have a structured top/bottom i.e. dominant/submissive relationship, but are referred to as seme and uke.