Tropical Storm Zeta was declared a tropical depression, making it the record-breaking 28th tropical cyclone of the 2005 Atlantic hurricane season, the most active in recorded history until 2020.

Tropical Storm Zeta was a very late-developing tropical storm that formed in the central Atlantic Ocean during the 2005 Atlantic hurricane season, forming one month after the season's official end. Becoming a tropical depression on December 30, and intensifying the following day into the season's 28th storm, Zeta continued into January 2006. It was one of only two Atlantic tropical cyclones to span two calendar years.

A tropical cyclone is a rapidly rotating storm system with a low-pressure area, a closed low-level atmospheric circulation, strong winds, and a spiral arrangement of thunderstorms that produce heavy rain and squalls. Depending on its location and strength, a tropical cyclone is called a hurricane, typhoon, tropical storm, cyclonic storm, tropical depression, or simply cyclone. A hurricane is a strong tropical cyclone that occurs in the Atlantic Ocean or northeastern Pacific Ocean. A typhoon is the same thing which occurs in the northwestern Pacific Ocean. In the Indian Ocean and South Pacific, comparable storms are referred to as "tropical cyclones". In modern times, on average around 80 to 90 named tropical cyclones form each year around the world, over half of which develop hurricane-force winds of 65 kn or more.

The 2005 Atlantic hurricane season was a record-breaking, devastating and deadly Atlantic hurricane season. It is the second-costliest hurricane season, just behind the 2017 season. It featured 28 tropical and subtropical storms, which was the most recorded in a hurricane season until the 2020 season. The United States National Hurricane Center named 27 storms, exhausting the annual pre-designated list, requiring the use of six Greek letter names, and adding an additional unnamed subtropical storm during a post-season re-analysis. A record 15 storms attained hurricane status, with maximum sustained winds of at least 74 miles per hour (119 km/h). Of those, a record seven became major hurricanes, rated Category 3 or higher on the Saffir–Simpson scale. Four storms of this season became Category 5 hurricanes, the most of any season on record.