partner
n. countablen. someone you do an activity with, like a person you work with or someone you are in a relationship with.
n. one of two or more people engaged in a shared activity, business venture, or romantic relationship.
She is my business partner and we share an office.
The teacher asked every student to find a partner for the science experiment.
While the firm began as a solo practice, it eventually expanded to include a junior partner who managed the litigation department.
From Middle English partener, partiner, alteration (due to Middle English part) of Middle English parcener, from Old French parçonier, parçonneour (“joint heir”) from parçon (“partition”), from Latin partītiōnem, singular accusative of partītiō (“portion”). The word may also represent Old French part tenour (“part holder”). Compare also Middle English partifere (“partner”), partifelewe (“partner”). Doublet of parcener.
Commonly used with the preposition 'with' or 'of' depending on the context of the relationship.