This article's content is accurate to the most recent public game version, 0.4.8d.
"Drains MP from one enemy. The infusion of fresh magic also raises Veronica's MAT for 1 turn."
Cost: 10 TP
Source: Veronica - Starting skill.
Mana Theft is a skill that deals 
Absorb type MP drain to a single enemy target, restoring the user's MP by the amount drained and giving MAT Up for one turn. Despite being considered a physical attack this skill scales off of the user's MAT and target's MDF.
Veronica's naturally low MAT generally leaves her at a serious disadvantage when dealing with physical-resistant foes. Buffing her spells with Mana Theft helps make her a more significant combatant in those battles.
The following enemies can cast Mana Theft: