What does the term Form Factor mean?

Form factor is a term used to describe the physical size and shape of a piece of computer hardware. The term is common within the IT industry and is used to identify how a piece of computer hardware should be installed: rack mount or tower form factor. If the device has a rack mount Form Factor a height will usually also be quoted in ‘U’. In a 19inch rack mount format 1U= 1.75inches or 44.45mm.