Fort (also called fortress or fortification) is any small work of fortification, permanent or temporary, that defends a pass or constitutes part of a system. According to its layout, object, layout or capacity, the fort is bastioned, isolated, open, advanced, closed, star-shaped, prominent, independent, etc.[1]​.

As an adjective, it is applied to what has strength and resistance, to what is robust. As a noun, it is a generic name for any small fortification work, whether permanent or temporary, but particularly for the latter, which defends a pass or constitutes part of a system.

Within military engineering, forts are a specific defensive typology. Its main tactical idea is to establish a "defensive curtain" known in nomenclature as a "Fort system." The military garrisons in charge of protection and surveillance are quartered there.