a fervent and even militant proponent of something
"The activist became such a fervent partisan of environmental reform that they organized protests outside every factory in the city."
an ardent and enthusiastic supporter of some person or activity
"The partisan cheered loudly whenever her favorite team scored a goal."
An adherent to a party or faction.
"The ancient warrior wielded his partisan to strike down the charging enemy with its distinctive double-edged, leaf-shaped head."
A long-handled spear with a triangular, double-edged blade having lateral projections, in some forms also used in boar hunting.
In plain English: A partisan is someone who strongly supports only one side of an argument or group, often ignoring other viewpoints.
"The partisan argued that his team was always right and everyone else was wrong."
Usage: The noun form refers specifically to the historical weapon known as a partisan spear and should not be confused with the adjective meaning a political supporter. When discussing modern politics, always use the adjectival sense rather than attempting to apply the obsolete weaponry definition.
Serving as commander or member of a body of detached light troops.
"The partisan led his small band of guerrilla fighters through the dense forest to ambush the enemy supply line."
In plain English: Partisan means being strongly loyal to one particular group, party, or side and often ignoring fairness toward others.
"The partisan judge ruled heavily in favor of his own party's candidate."
The word partisan comes from French and Italian, originally meaning a defender or adherent of a political party. Its later use to describe a guerrilla fighter emerged around 1690, influenced by similar terms in Slavic languages that also referred to irregular combatants.