What does "in persona Christi Capitis" mean? The Church says that the Priest acts "in persona Christi Capitis". 
Where is this term taken from?
It means: &quot;in the person of Christ the Head [of the Church]&quot;.
Christ is the High Priest, in Whose person all other ordained priests act when they offer the Holy Sacrifice of the Mass.
When a Catholic priest consecrates the host and wine, he truly identifies Himself with Christ when he says &quot;This is my body.&quot; and &quot;This is my blood.&quot;
St. Robert Bellarmine writes (Controversiis t. 2, De Missa, 1, c. 27), as quoted in Pope Pius XII's encyclical on the sacred liturgy, Mediator Dei §86:

The sacrifice is principally offered in the person of Christ. Thus the oblation that follows the consecration is a sort of attestation that the whole Church consents in the oblation made by Christ, and offers it along with Him.
Sacrificium, inquit, in persona Christi principaliter offertur. Itaque ista oblatio, consecrationem subsequens, est quaedam testificatio, quod tota Ecclesia consentiat in oblationem a Christo factam, et simul cum illo offerat.