Lemon cars have either shit reliability to the extreme or cars with a glaring insane flaw such as the infamous Ford pinto that would explode into flames if it got rear ended.
Good example is the 2016-2021 Honda Civic! Tons of electrical issues, and refrigerant that is incompatible with the ac condenser, so you sink thousands into your AC for it to stop working after a couple of months.
i had a CR-V which had a really great electrical issue where it would drain the battery if you didn't start it every day. The dealer suggested that i should have somebody go to the airport and start the car for me every day while i was working out of town 0_o
This may or may not be an official definition, but in my retail experience a lemon anything is something that has been returned as defective three or more times.
I have no idea the formal definition but locally around me it refers to a car that has been sold for far too much due to the near guaranteed chance it will break down within the next dozen drives.