A hajduk is a type of irregular infantry found in Central, Eastern, and parts of Southeast Europe from the late 16th to mid 19th centuries. They have reputations ranging from bandits to freedom fighters depending on time, place, and their enemies.

Irregular military is any non-standard military component that is distinct from a country's national armed forces. Being defined by exclusion, there is significant variance in what comes under the term. It can refer to the type of military organization, or to the type of tactics used. An irregular military organization is one which is not part of the regular army organization. Without standard military unit organization, various more general names are often used; such organizations may be called a troop, group, unit, column, band, or force. Irregulars are soldiers or warriors that are members of these organizations, or are members of special military units that employ irregular military tactics. This also applies to irregular infantry and irregular cavalry units.