incidental Definition

1. Happening, etc by chance in connection with something else, and of secondary or minor importance.

Example: incidental expenses
Thesaurus: accidental, chance, random, occasional, casual, fortuitous, odd, inconsequential, irrelevant, nonessential, trivial, slight, minor.
2. Occurring or likely to occur as a minor consequence of it.

Form: incidental to something (usually)
3. Following or depending upon it, or caused by it, as a minor consequence.

Thesaurus: contingent, attendant, subordinate, secondary, accompanying, related.
Form: incidental on something (usually)
Form: incidental upon something

1. Anything that occurs incidentally.
2. Minor expenses, details, items, etc.

Thesaurus: odds and ends, extras, accessories; Antonym: essentials.
Form: incidentals

By the way, parenthetically ― often used to introduce a sentence or phrase describing something which is, or might seem to be, a secondary or minor but related matter.
Thesaurus: by the way, parenthetically, by the by, en passant, in passing.

In an incidental manner.