A lemon is a slang term for a used car that doesn't work or is of poor condition. It's slangy but not too informal so you can use it officially; but it is only used to describe cars that are (deceptively) run-down.

Lemon is also used for a new car that was built improperly and has too many flaws to be resolved. Some states in the USA have what are called "lemon laws" that afford the buyer certain recourses to having bought a lemon, if they can prove it. It is common folk lore to buy a car that was built at the factory on Tuesday, Wednesday, or Thursday when the assembly workers are more focused on their work and less so on their previous or upcoming weekend.

the term is not that informal -- i've heard it used by a judge in a court of law.