IC is usually meant in contrast to manager, meaning ICs are the engineers without anyone reporting to them.

The difference between "not a manager" and "working on their own part of the code" is important when you're talking about positions with a lot of seniority like staff or principal engineers, because those ICs are still expected to work on massive projects involving a lot of cat-herding and leadership that touch large chunks of a company's code, they just aren't doing it as the manager of a team.