Anything (such as a piece of furniture, or a software feature) that is built in, not added as an extra.
Being an essential and permanent part of something.
Being an included feature that normally comes as an extra.
Constructed as a non-detachable part of a larger structure.
Entering or approaching, prior to reflection (more frequently incident).
Occurring by chance.
Existing tangentially, being a byproduct, a tangent, or a likely consequence.
Loosely associated; of limited relevance except indirectly; only accidentally related.
Something that is incidental.
Minor items, not further defined. Incidental expense.