It can be a pretty loose term.
Technically anyone is interfering with play because defenders are thinking about where they are and positioning themselves in response to where everyone else is on the field.
Although
@socal lurker already said this, I want to reiterate it because it's an important point, especially when we're on a refereeing forum and discussing the technicalities of the offside law.
Technically, and under the Laws of the Game, "interfering with play" is not a loose term. It is clearly defined as one thing and one thing only - interfering with play means touching the ball.
It's a misleading term (and I've communicated my thoughts on this to the IFAB more than once) because in terms of natural language usage, it is a loose term leading to much misunderstanding and moreover as the laws stand, it's unnecessary.
It was needed once, when the main body of the law just said, "interfering with play" and the definition was given only later on in the "Interpretation of the Laws of the Game and Guidelines for Referees" section but now that the law itself explains what is involved, the phrase "interfering with play" could simply be removed. No change in meaning would occur and a much misunderstood term would be gone, making things much simpler and easier to interpret.
The same goes, incidentally for "gaining an advantage." Much better (IMHO) to simply remove it and leave only the definition of what it means.
