What Is Host in the Bible?

Object

The thin, round unleavened bread that is consecrated at Mass. Once consecrated, it is the Body of Christ.

It can mean

What it is made of: wheat flour alone, recently made, and without yeast.
At Communion: the priest shows it and says: “The Body of Christ.” The one receiving answers: “Amen.”
In the Spanish Bible of 1836: “hostia” means victim, what was offered to God in sacrifice. That is why it says that Christ offered “one single host” (the Douay-Rheims says “one sacrifice”).

When the Host is kept in the tabernacle, people adore it outside of Mass.

Luke 22:19; Matthew 26:26; Hebrews 10:12

Luke 22:19

And taking bread, he gave thanks, and brake; and gave to them, saying: This is my body, which is given for you. Do this for a commemoration of me.

Matthew 26:26

And whilst they were at supper, Jesus took bread, and blessed, and broke: and gave to his disciples, and said: Take ye, and eat. This is my body.

Hebrews 10:12

But this man offering one sacrifice for sins, for ever sitteth on the right hand of God,

Douay-Rheims Bible (Challoner revision), public domain.

Source: Catechism 1376 and 1378; GIRM 161 and 320; RAE, “hostia”