Advertising and promotional slang for a DEad ceLEBrity, especially one whose image can be used in advertising and, through photoshop or other c.g.i. process, can be seen to advocate contemporary products or causes.
Right to use such images can be bought and sold, much like the endorsements and images of still-alive celebrities.
Who's our agency's biggest deleb? It's not James Dean or Marilyn Monroe, it's Albert Einstein!