Terrain Bonus

Terrain bonus is an important concept in many games in the Fire Emblem series, it refers to the various effective stat increases one recieves by standing on a particular type of terrain.

Typically in most fire emblem games terrain bonus decreases the hit chance, damage and magical damage of opponents attacking the occupent of the affected square though under some circumstances it can even add to the hit chance and damage of the square's occupant.

There are also related effects which while not strictly a terrain bonus are worth mentioning. These include some squares, usually on lava themed maps in various games which deal damage each turn to whomever is standing on them. These are usually graphically distinct, having a deep red color in most games.

Finally, in Radiant Dawn, one can recieve a bonus similar to a terrain bonus by being in a higher square than an opponent during combat. This to my knowledge is the debut of square height affecting combat in the series.