1. Candle: This is the most common meaning of "bougie" and refers to a wax cylinder with a wick that is burned to produce light.
2. Spark Plug: This is a more technical meaning, referring to the part of an internal combustion engine that ignites the fuel-air mixture. This meaning comes from the fact that spark plugs were originally made from porcelain and resembled a candle in shape.
The context will usually make it clear which meaning is intended.
It's important to note: While "bougie" is a French word, it's not always used in the same way in English. In English, "bougie" is sometimes used as a slang term to describe something considered trendy, stylish, or expensive. This usage is not directly related to the French meaning of the word.