Eraldo Coil (エラルド・コイル, Erarudo Koiru) is the name of the second most famous detective in the world. The actual Coil is no longer active at the beginning of the series, and L uses their name as an alias for some of his cases.
L tells the Task Force that he uses the alias as a method to further conceal his true identity, since anyone trying to find L's identity is likely to contact Coil or Denueve, the second and third most famous detectives in the world. This happens during the Kira Investigation when the Yotsuba Group contacts Coil to find L.
Aiber poses as Coil to trick the Yotsuba Group. As Coil, he sits with the Yotsuba members during Misa Amane's interview and questions her about L suspecting her of being the second Kira. Their questions are rehearsed, and the members believe it.
In Death Note: Another Note: The Los Angeles BB Murder Cases, it is revealed that Eraldo Coil was an actual person before giving up his name to L.
Despite the Yotsuba Group being unaware that Aiber was not actually Coil, it is said in Death Note: Another Note that it is common knowledge that L, Eraldo Coil and Denueve were one and the same.