A person or company added by endorsement to another party's insurance policy, so that policy now covers them too for claims arising from the named insured's work.
Being named as an Additional Insured (AI) on someone else's policy gives you actual coverage under their policy — not just evidence of theirs. It's how a GC pushes liability for a sub's mistakes onto the sub's insurance instead of their own.
Adding an AI requires an endorsement to the underlying policy, not just a COI. The COI may indicate AI status, but the legal weight comes from the endorsement form (typically CG 20 10 or CG 20 38 in standard ISO forms).
Smart GCs require every sub to add the GC as an Additional Insured on the sub's GL policy, with primary and non-contributory wording, before issuing a PO.