Level

Level is a measure of the proficiency a character has in their current class.

Maximum Level
The maximum Level for a character is usually 20. In Fire Emblem: Seisen no Keifu the maximum Level is 30. For laguz units in Fire Emblem: Radiant Dawn, the maximum Level is 40. In Fire Emblem: Shadow Dragon Marth, Thieves Manakete and Ballisticians can reach level 30.

Changes In Level
Level increases when a character attains 100 Experience Points. This triggers an event known as a Level Up. Aside from on characters with growths higher than 100%, Level is the only stat guaranteed to rise on Level Up (except in Fire Emblem: Radiant Dawn, where growths are programmed so you get at least one every time you level up.

There is an item called Juna Fruit in Fire Emblem: The Sacred Stones which can be used to decrease a unit's level. This item is not available ingame, however, and can only be acquired through cheating.

Effects of Level
Most characters can only promote once they are above a certain Level. In most games, the Level required is 10. Seisen no Keifu on the other hand allows promotion from Level 20.