Carol Ann Susi -- one of the central characters on "The Big Bang Theory" -- died Tuesday after battling cancer ... according to execs on the hit show.

Susi played Howard Wolowitz's mom who almost NEVER appeared on camera -- but whose voice was heard in just about every episode.

'Big Bang' executive producer Chuck Lorre said ... "The Mrs. Wolowitz character became a bit of mystery throughout the show's eight seasons. What was not a mystery, however, was Carol Ann's immense talent and comedic timing."

Susi's rep described her cancer as "aggressive" ... and added it was a brief battle.  

Wolowitz's mother was only seen once on 'Big Bang' -- and it was just a quick glimpse during Howard's wedding. It's unclear how Lorre and producers will handle Susi's death on the show. Her character's had a central plot line this season.

Carol Ann Susi was 62.