1. Situated or running side by side; parallel.
2. Coinciding in tendency or effect; concomitant or accompanying.
3. Serving to support or corroborate: collateral evidence.
4. Of a secondary nature; subordinate: collateral target damage from a bombing run.
5. Of, relating to, or guaranteed by a security pledged against the performance of an obligation: a collateral loan.
6. Having an ancestor in common but descended from a different line.
1. Property acceptable as security for a loan or other obligation.
2. A collateral relative.
[Middle English, from Medieval Latin collaterlis : Latin com-, com- + Latin latus, later-, side.]
(definition from The Free Dictionary)