Silent clock refers to an event on the television show 24.

As a part of the show's "real-time" format, a digital clock in yellow typeface is displayed at the beginning and ends of each commercial break as well as at the beginning and end of each episode, in order to keep the viewer informed of the current time within the 24 universe. The clock is normally accompanied by a distinct "tick-tock" sound.

Explanation

A 'silent clock' has, over the course of the series, become a way to honor the deaths (typically) of significant characters. To many fans it is considered the highest honor a character can recieve from the show upon dying, and has thus only been used five times out of the countless times the clock has been seen.

Instances

spoilers below

Day 1: A silent clock occured as the last clock displayed that season, right after the death of Teri Bauer.

Day 2: A silent clock was shown after George Mason's departure from CTU. Although he had not yet died from the effects of radiation poisoning, the clock foreshadowed the inevitable. The silent clock was also used at the end of that day, after the terrorist Mandy's attack on President David Palmer. It was unsure at the time if the President was to survive the attack.

Day 3: At 6:59 AM, Jack Bauer was forced to kill Ryan Chapelle. A silent clock honored his death.

A silent clock did not occur on the fourth day.

Day 5: Edgar Stiles, while looking for a missing CTU agent before the Russian terrorist's gas attack on CTU itself, was unable to get to a secure area before he succumbed to the effects of the nerve gas released in the building. As a horrified Chloe O'Brien looked on, Edgar fell, and his death was the fourth to be given a silent clock.