A PASSEX is a U.S. Navy term for a "passing exercise". Almost any US military

Military

A military is an organization authorized by its greater society to use lethal force, usually including use of weapons, in defending its country by combating actual or perceived threats. The military may have additional functions of use to its greater society, such as advancing a political agenda e.g...

 term in all capitals ending in ...EX is some type of an exercise.

A passing exercise is done between any two navies

Navy

A navy is the branch of a nation's armed forces principally designated for naval and amphibious warfare; namely, lake- or ocean-borne combat operations and related functions...

 for the official reason of ensuring that the two different navies are able to communicate and cooperate in times of war

War

War is a state of organized, armed, and often prolonged conflict carried on between states, nations, or other parties typified by extreme aggression, social disruption, and usually high mortality. War should be understood as an actual, intentional and widespread armed conflict between political...

 or humanitarian relief. Common drills might include flashing light drills, semaphore

Semaphore

A semaphore telegraph, optical telegraph, shutter telegraph chain, Chappe telegraph, or Napoleonic semaphore is a system of conveying information by means of visual signals, using towers with pivoting shutters, also known as blades or paddles. Information is encoded by the position of the...

 drills, and flaghoist drills. In more modern times, navies within a very close alliance might include much more electronic and digital cooperation including using electronic communications and target acquisition. Some exercises between the French and US navies (particularly in support of the war in Afghanistan

Afghanistan

Afghanistan , officially the Islamic Republic of Afghanistan, is a landlocked country located in the centre of Asia, forming South Asia, Central Asia and the Middle East. With a population of about 29 million, it has an area of , making it the 42nd most populous and 41st largest nation in the world...

) have included carrier

Aircraft carrier

An aircraft carrier is a warship designed with a primary mission of deploying and recovering aircraft, acting as a seagoing airbase. Aircraft carriers thus allow a naval force to project air power worldwide without having to depend on local bases for staging aircraft operations...

-based fighter-bombers "touching down" (landing then taking off) on each others carriers in order to ensure interoperability.

While the official reason of a PASSEX is to practice cooperation, various unofficial reasons might be to "show the flag" (show the power of a nation through a public display of naval power) or other political or diplomatic

Diplomacy

Diplomacy is the art and practice of conducting negotiations between representatives of groups or states...

reasons.

Recently Indian navy undertook a (PASSEX) exercise with Turkish navy on 10th and 11th july 2011.