A "partisan" is best described as:

A "partisan" is best described as a biased supporter, which aligns perfectly with the definition of the term. The word originates from the context of political or social issues where it refers to an individual who shows a strong allegiance to a particular party or group. This allegiance often leads them to support the views and policies of that party or group without consideration for opposing opinions or neutral perspectives.

This characteristic of being biased implies that the individual is not merely supportive in a balanced or fair manner; instead, they actively promote their group's interests, often at the expense of objectivity. By understanding "partisan" in this way, one can grasp its usage in contexts such as political discourse, campaign strategies, and debates, where the influence of partisan beliefs can significantly shape opinions and decisions.