Sol (太陽 Taiyō, lit. Sun; also known as Sun Sword [日曜剣 Nichiyō ken] in the Jugdral series, Solaire in the French version, Helios in Spanish version) is a mastery skill that was introduced in Fire Emblem: Genealogy of the Holy War. It allows the user to recover HP equal to the amount of damage done to the enemy.
This effect is slightly altered in Fire Emblem: Radiant Dawn, where it also grants the user the ability to inflict triple damage on the enemy upon activation.
In Fire Emblem Awakening and Fire Emblem Fates, this effect only heals half of the damage inflicted on the opponent in HP. For example, if 16 damage is inflicted on an enemy with only 8 HP left, the HP restored will be 4.
"Sol" probably derived from the word sol, which means sun in several languages, specially Latin-based/Romance ones: Latin itself, Spanish, Catalan, Portuguese, etc. It could also be derived from the word soleil, which also mean sun in French (another Romance language).