AKA a Hose bib, Hosecock, Bibcock, Spigot, and an exterior faucet.
A sillcock is an an exterior faucet, usually used for attaching a hose. In America the terms 'sillcock' and 'hose bib' are commonly used by plumbers to refer to any outdoor faucet that is attached to the foundation of your house. The terms are often used interchangeably, but some will claim that there is an important difference between the two. If someone claims that there is a difference between a sillcock and a hose bib, generally they will use hose bib as a general term, and sillcock to refer to a specific type of hose bib.
The two most common features that are said to define a sillcock are either that:
- Sillcocks have a flange or 'skirt' that allows it to be screwed firmly to the house, and covers the rim of the hole that the pipe passes through.
- Sillcocks are 'Freezeless Faucets', a type of hose bib in which the valve is not actually in the faucet itself, but at the end of a stretch of pipe (6″ to 12″) that protrudes back into the house. Thus the faucet and parts of the pipes that are most likely to freeze will drain completely when the faucet is off.
When in doubt, ask your plumber/hardware store to define their terms.
Etymological note: In this case 'sill' refers to the foundation of a house (as a window sill refers to the foundation of a window), and 'cock' refers to faucet.