WHAT IS IT?  A limit much like a PEL but not a legal limit.  TLVs are set by a professional organization (safety and health professionals).  TLVs are sometimes more strict than the PELs and are starting to assume prominence as "standard prudent practice" in spite of not being set by OSHA.

The PEL of acetone is 1000 ppm (8-h TWA)

The TLV of acetone is 500 ppm (8-h TWA)