Gilyard passed away after a long battle with an unspecified illness, according to media reports. His death was announced Monday by the University of Las Vegas, Nevada, where he had worked since 2006 as a Fine Arts professor.

His 30-year career in Hollywood landed him major roles in popular TV series, from the '80s and '90s ... notably, "Matlock," and "Walker, Texas Ranger." He worked with stars like Andy Griffith and Chuck Norris. He also scored his first big screen role as Sundown in "Top Gun."

Gilyard's breakout role was playing Theo in the OG "Die Hard." Theo was the sardonic tech wizard who took over a Los Angeles skyscraper with terrorists and hacked into a vault, attempting to steal $640 million in bearer bonds. In one scene, the terrorists bomb an LAPD vehicle and Theo famously cackles, "Oh my god, the quarterback is toast!"

UNLV Dean Nancy J. Uscher remembered Gilyard this way ... "It is with profound sadness that I share this news. His students were deeply inspired by him, as were all who knew him. He had many extraordinary talents and was extremely well-known in the university through his dedication to teaching and his professional accomplishments."

Uscher went on ... "His generosity of spirit was boundless - he was always ready to contribute to projects and performances however possible."

Gilyard was 66.