[f., pl. “Mezuzot”]; literally, the door-post of a house. In the context of Torah commands, it has the related meaning of a scroll affixed to the door-posts of one’s house and the rooms within one’s house. The Command to do so is found in Devarim 6:9, “And these words that I command you this day…you shall write them on the door-posts of your house and on your gates.”
This Command commemorates the time in ancient Egypt that HaShem commanded the Jewish People to mark their door-posts with the blood of a lamb, the animal that was worshipped by the Egyptians, and that was used as a sacrifice by the Jewish People. So that when He brought the Tenth Plague, the Plague of the Killing of the First-Born upon the Egyptians, He would “Pass Over” (hence the name “Passover,” or Pesach, for the Holiday that celebrates the Exodus) the houses whose door-posts were marked.
The “Mezuzah” is a small scroll of parchment on which are written two Biblical passages: they are, “Hear O Israel, the L-rd our G-d, the L-rd is One…” (Devarim 6:4-9) and “And it shall be that if you carefully observe My Commandments …” (Devarim 11:13-21)
Before the Mezuzah is affixed to one’s door-post, one recites the following Berachah: “Blessed are You, L-rd our G-d, King of the universe, Who has sanctified us with His Commandments and has commanded us to Affix the Mezuzah.”
The Mezuzah, in its case, is then nailed or screwed or glued to the right side of the door, in the upper third part of the door-post, leaning inward towards the interior of the house or the room.