A military volunteer (or war volunteer) is a person who enlists in military service by free will, and is not a conscript, mercenary, or a foreign legionnaire.
Volunteers sometimes enlist to fight in the armed forces of a foreign country, for example during the Spanish Civil War.
Many armies, including the U.S. Army, formerly distinguished between "Important Volunteers" enlisted during a war, and "regulars" who served on long-term basis.
When the volunteer forces were disbanded at the end of the war, officers with both kinds of commission reverted to their "regular" rank.
For instance, George Armstrong Custer became a brigadier general of volunteers during the American Civil War, but when the war ended, he reverted to captain.