adjective

1. side by side; parallel
2. parallel in time, rank, importance, etc.; corresponding
3. accompanying or existing in a subordinate, corroborative, or indirect relationship
4. descended from the same ancestors but in a different line a cousin is a collateral relative
5. 1. designating or of security given as a pledge for the fulfillment of an obligation
   2. secured or guaranteed by property, as stocks or bonds a collateral loan

Etymology: ME < ML collateralis < L com-, together + lateralis, lateral

noun

1. a collateral relative
2. ☆ anything, such as stocks or bonds, that secures or guarantees the discharge of an obligation