The difference between Certain and Incidental

When used as nouns, certain means something certain, whereas incidental means minor items, not further defined. incidental expense.

When used as adjectives, certain means sure, positive, not doubting, whereas incidental means loosely associated.

Certain is also determiner with the meaning: having been determined but not specified.

Certain is also pronoun with the meaning: unnamed or undescribed members (of).

check bellow for the other definitions of Certain and Incidental

Certain as an adjective:

Sure, positive, not doubting.

Examples:

"I was certain of my decision."
Certain as an adjective (obsolete):

Determined; resolved.
Certain as an adjective:

Not to be doubted or denied; established as a fact.
Certain as an adjective:

Actually existing; sure to happen; inevitable.

Examples:

"Bankruptcy is the certain outcome of your constant gambling and lending."
Certain as an adjective:

Unfailing; infallible.
Certain as an adjective:

Fixed or stated; regular; determinate.
Certain as an adjective:

Not specifically named; indeterminate; indefinite; one or some; sometimes used independently as a noun, and meaning certain persons; see also "one".

Certain as a pronoun (with ''of''):

Unnamed or undescribed members (of).

Examples:

"There where serious objections to certain of the proposals."

Incidental as an adjective:

Loosely associated; existing as a byproduct, tangent, or accident; being a likely consequence.

Examples:

"That character, though colorful, is incidental to the overall plot."
Incidental as an adjective:

Occurring by chance
Incidental as an adjective (physics, of radiation):

Entering or approaching, prior to reflection (more frequently incident).

Incidental as a noun:

Minor items, not further defined. Incidental expense.

Examples:

"She's costing us a lot in incidentals."
Incidental as a noun:

Something that is incidental.