Heavy infantry refers to heavily armed and armoured ground troops, as opposed to medium or light infantry, in which the warriors are relatively lightly armoured. As modern infantry troops usually define their subgroups differently (such as by weapon or type of attack), 'heavy infantry' almost always is used to describe pre-gunpowder age troops.
In ancient Greece the Hoplite was a common form of heavy infantry. All hoplites had a shield and spear, and perhaps a helmet as well. Wealthier hoplites were able to afford bronze breastplate or linothorax armor, while poorer hoplites wore little to no armor. The hoplite armor and shield were designed to block arrows and blows from spear points and swords. Hoplites would act as both a city watch and as an army in the field. Hoplites were thought of as a force to be reckoned with because they would form a phalanx, a tight band of spearmen, which aided them against lighter infantry and cavalry.