noun.property (such as securities) pledged by a borrower to protect the interests of the lender

• A collateral inherited the estate.
• Attendees can expect to make approximately 50 new business contacts and should … have an ample supply of business cards, marketing collateral and anything else to help potential leads remember them.

noun.a collateral relative

noun.a branch of a bodily part (such as a vein)

adjective.accompanying as secondary or subordinate: concomitant

• digress into collateral matters
• no direct objection, but a few collateral ones

adjective.indirect

• Collateral evidence was presented at the trial.
• Brothers, cousins, uncles, and nephews are collateral kinsmen.

adjective.serving to support or reinforce: ancillary

• collateral states like Athens and Sparta
• a collateral loan secured by stocks and bonds deposited with the lender