PHB, page 275
Shield
1-level Abjuration
Casting Time: 1 reaction, which you take when you are hit by an attack or targeted by the magic missile spell
Range/Area: Self
Components: V, S
Duration: 1 round
An invisible barrier of magical force appears and protects you. Until the start of your next turn, you have a +5 bonus to AC, including against the triggering attack, and you take no damage from magic missile.
Available for: Sorcerer, Wizard