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A moment, as seen in Lord of the Rings which it is named after, where a small army is being slaughtered by a giant army, and a single character (usually a hero/captain or something) appears followed by an army. The cavalry then, with the combined efforts of the original army, defeat/drive back the much larger army with ease.

There are multiple variants of this, for example, a small group of people (20 or so) are attempting to attack a large force of people (anywhere from 100-1000) and they say something along the lines of "we need more men" Then a main character of the story (usually someone who left the group) shows up with a large group of men to reinforce the small group so they may attack and win.
Rohanian Cavalry Moment Example: *SPOILERS* In The Lord of the Rings where Aragorn arrives at Minas Tirith with the army of Rohan and drives the army of Sauron away with the combined efforts of the armies of Gondor and Rohan.
by Fox Named Baron August 07, 2011