The so called "momentum" (or scripting/handicap) feature changes the abilities of the players on the pitch from game to game in a seemingly random manner, usually giving one of the teams a major advantage, while the other has an increased chance of failed passes or hitting the post.
It was heavily featured in FIFA 13 and FIFA 14, which lead to a lot of discussion and is dreaded by many players, but EA apparently wants to keep it in to make matches more unique and give weaker players a better chance for winning once in a while against stronger opponents.
Obviously this is a bad idea for online play, especially competitive play in leagues etc.
Is there any official information if momentum is included in FIFA 15 for online modes like Seasons?
The problem with simply checking it by playing is that you usually have to play a lot of matches to be able to tell that momentum exists (and you are not simply having a bit of bad luck).