Epitaxy refers to the deposition of a crystalline overlayer on a crystalline substrate.

The overlayer is called an epitaxial film or epitaxial layer. The term epitaxy comes from the Greek roots epi (ἐπί), meaning "above", and taxis (τάξις), meaning "an ordered manner". It can be translated as "arranging upon". For most technological applications, it is desired that the deposited material form a crystalline overlayer that has one well-defined orientation with respect to the substrate crystal structure (single-domain epitaxy).