A tercio is a term used by the Spanish army to describe a group of almost 3000 infantry. Tercios were well known for their superiority and effectiveness in combat during 16th and 17th centuries in Europe. The tercios were mostly formed by and professional soldiers hardened in many battles. The tercios were feared by the enemy troops because of the legendary determination of its soldiers in combat, and the prospect of being thrown to battle against the Spanish tercios even led to desertions in the rival forces several times.

A tercio consisted of 1500 pikemen and about the same number of musketeers.

The pikemen always stayed together in one large square (carré ).

The musketeers were split up in four groups (mangas) and deployed relative to the carré, for instance one manga at each corner.

Tercios were eventually replaced with battalions and regiments.

Today the Spanish Foreign Legion still uses the tercios as its largest unit designation.