Colitis is an inflammation of the colon. Colitis may be acute and self-limited or long-term. It broadly fits into the category of digestive diseases.
In a medical context, the label colitis (without qualification) is used if:

The cause of the inflammation in the colon is undetermined; for example, colitis may be applied to Crohn's disease at a time when the diagnosis is unknown, or
The context is clear; for example, an individual with ulcerative colitis is talking about their disease with a physician who knows the diagnosis.