Chumash (Book)
The Chumash is a term for each of the five books of the Torah.
Name
The Torah is divided into five books: Sefer Bereshit (Genesis), Sefer Shemot (Exodus), Sefer Vayikra (Leviticus), Sefer Bamidbar (Numbers), and Sefer Devarim (Deuteronomy).
Each of these five is called a Chumash, meaning "one of five." It is sometimes printed as one book, and sometimes as five separate parts.
Holiness
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The Chumash has a special holiness, and therefore no other book (even Tanya) should be placed on top of the Chumash.
The Rebbe, when entering 770, was observed many times removing various books from on top of the Chumash, or instructing others to remove them.
Although one should not place a Tanya on a Chumash, one should also not place a Chumash on a Tanya, since it [Tanya] is also considered a "Chumash" of Chassidic teachings.
Further Reading
Rabbi Y.S. Ginzburg. Placing holy books on top of each other (including the Rebbe's customs) - Chabad website