The Mk 2 defensive hand grenade is a fragmentation

Fragmentation (weaponry)

Fragmentation is the process by which the casing of an artillery shell, bomb, grenade, etc. is shattered by the detonating high explosive filling. The correct technical terminology for these casing pieces is fragments , although shards or splinters can be used for non-preformed fragments...

 hand grenade (sometimes written Mk II) used by the U.S. armed forces

Military of the United States

The United States Armed Forces are the military forces of the United States. They consist of the Army, Navy, Marine Corps, Air Force, and Coast Guard.The United States has a strong tradition of civilian control of the military...

 during World War II

World War II

World War II, or the Second World War , was a global conflict lasting from 1939 to 1945, involving most of the world's nations—including all of the great powers—eventually forming two opposing military alliances: the Allies and the Axis...

 and in later conflicts including the Vietnam War

Vietnam War

The Vietnam War was a Cold War-era military conflict that occurred in Vietnam, Laos, and Cambodia from 1 November 1955 to the fall of Saigon on 30 April 1975. This war followed the First Indochina War and was fought between North Vietnam, supported by its communist allies, and the government of...

. The Mk II was standardized in 1920 replacing the Mk I

Mk 1 grenade

The Mk 1 grenade is a fragmentation hand grenade used by American forces during World War I. According to its designers, it was to be the "simplest", yet most "fool-proof", grenade ever made...

 of 1917. It was phased out gradually, the U.S. Navy

United States Navy

The United States Navy is the naval warfare service branch of the United States Armed Forces and one of the seven uniformed services of the United States. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest navies combined. The U.S...

 being the last users. It was replaced by the M26-series and later M61

M61 grenade

-Description:The M26 is a fragmentation grenade developed by the United States military. Its distinct lemon shape led it to being nicknamed the "lemon grenade"....

 and M67 grenade

M67 grenade

The M67 grenade is a fragmentation hand grenade used by the United States Military and Canadian Forces, where it is referred to as the C13. The M67 is a replacement for the M61 grenade used during Vietnam and the older Mk 2 "pineapple" grenade used since World War II.-Composition:The M67 Grenade...

s. On 2 April 1945 the Mk II and Mk IIA1 were redesignated the Mk 2 and Mk 2A1.
The Mk 2 was commonly known as a "pineapple" grenade, because of its shape and structure.