Mezuzah literally means a doorpost in the Hebrew language. (plural: "mezuzot").
This refers to one of the 613 mitzvot/613 commandments in Judaism which requires a small parchment ("klaf") inscribed with two sections from the Torah's Book of Deuteronomy: 6:4-9 and 11:13-21. It is usually placed into a small case, after being carefully rolled up with the wording on the inside, then be affixed on the inside of the upper third of each doorpost and gate in a Jewish home, synagogues, and business (excluding bathrooms).
Thus the word ''mezuzah'' can refer to (one or all of):
Simply a doorpost of a permanent door, gate, or arch.
The special parchment with the required Hebrew alphabet/Hebrew inscriptions. (The parchment can be attached without being inside a box).
The small case or box that covers the parchment. (If the box does not have the rolled up parchment inside of it, it cannot serve the purpose of the mitzvah/commandment of ''mezuzah''.) The case generally features the Hebrew alphabet/Hebrew letter "Shin" inscribed on its upper exterior. The external artistic mezuzah cases are often given as gifts for weddings and other special occasions.
''Halakha'' ("Jewish law") prescribes in detail the affixing of ''mezuzot'' on doorposts. Since almost every Jewish home has a ''mezuzah'' on its front doorpost, it has historically been a means of recognition of a Jewish home.
The wording on the mezuzah's parchemnt is taken from the Shema Yisrael ("Hear O' Israel") prayer (similar to the parchments inside the Tefillin ("Phylactories").
