Before the priest performs the Eucharistic ceremony, he holds an object we call a 'wafer'. After the ceremony is complete, can we still correctly call the object a 'wafer'? Or can it only be correctly called the 'host'?
Fr. John Hardon, S.J., Modern Catholic Dictionary:
A victim of sacrifice, and therefore the consecrated Bread of the Eucharist considered as the sacrifice of the Body of Christ. The word ["host", with a lowercase "h"] is also used of the round wafers used for consecration. (Etym. Latin hostia, sacrificial offering.)
Prior to consecration, at the part of the Holy Sacrifice of the Mass called the offertory, the priest refers to the wafer of wheat as a host (lowercase "h"):
Súscipe, sancte Pater, omnípotens ætérne Deus, hanc immaculátam hóstiam…
Accept, O holy Father, almighty and eternal God, this unspotted host (victim)…
After consecration, at the Unde et mémores prayer, he says, while making five signs of the Cross with his right hand over the consecrated Host and chalice of His Blood:
…offérimus præcláræ maiestáti tuæ de tuis donis ac datis, hóstiam ✠ puram, hóstiam ✠ sanctam, hóstiam ✠ immaculátam, Panem ✠ sanctum vitæ ætérnæ, et Cálicem ✠ salútis perpétuæ.
…we offer unto Thy most excellent Majesty of Thine own gifts, bestowed upon us, a pure ✠ Host, a holy ✠ Host, an unspotted ✠ Host, the holy ✠ Bread of eternal life, and the Chalice ✠ of everlasting salvation.