An instrument that lets you measure the air pressure inside a vehicle tire. The cheapest versions (in the U.S.) tend to look like slightly fat ball-point pens (biros, for our friends Across The Pond). It has been correctly noted that the cheapest versions are also fairly inaccurate, although the ones that can sometimes be found built in to the compressors at service stations are usually much worse. Still, although it doesn't seem like it, correct tire inflation is an important factor in safety, tire wear and fuel efficiency - so purchasing a decent tire pressure gauge (and verifying its accuracy) can save you money and peace of mind in the long run.