Internal force (內功) is often translated as internal energy or internal power, etc. Some people will jump to equating this as cultivation. However, this is not the same as Chinese cultivation.

It is actually short for 內家功夫 which in Korean is 내가의 공부 which is basically the study of the “internal home” (literal translation).

This term is used both as a technique’s name and a measure of amount. The technique is one for generating strength through breathing. How much strength you gather and store is also referred to with this term. So you can say you “used up a lot of internal force.” More often than not, authors will refer to the amount of internal force being equivalent to XX number of years of training. Authors will have characters ingest elixirs in the form of pills or plants to boost the amount of internal force and give them a jumpstart of 2 years, 5 years, 15 years, etc. of training. These mostly come from the genre of martial arts novels that we call “fusion martial arts” in Korea.