055

055 is a numeric designation that can refer to several distinct concepts depending on context.

In mathematics, 55 is the tenth Fibonacci number and the tenth triangular number. It is the sum of the integers from 1 to 10, reflecting its status as a triangular number.

In chemistry, 55 is the atomic number of cesium (Cs), a soft, highly reactive alkali metal. Cesium has important applications in electronics, optics, and timing devices, and it is notable for its reactivity with water.

In modern military nomenclature, Type 055 refers to a class of large guided-missile destroyers built for the People's Liberation Army Navy (PLAN). Also known by NATO as the Renhai-class, these ships are characterized as multi-mission surface combatants with substantial vertical launch systems intended for air defense, anti-ship, and anti-submarine roles. The class is among the largest and most capable in Chinese naval service, with several ships commissioned or launched since the late 2010s.

Other uses of 055 include its use as a code, model number, or identifier in various contexts, such as product catalogs, transportation routes, or administrative designations. As a numeric string, it may appear in software, labeling schemes, or regional numbering systems.

See also: related numbers such as 5, 55, and 555, as well as entries for the element cesium and the PLAN Type 055 destroyer class.