The Japanese “no” particle. Used to indicate possession or a relationship between two things. For example:
Atemi No Waza. This might be interpreted as Atemi’s Tricks, or as Tricks of Atemi (the latter being more appropriate in this case). But “Neko no Jim-san” would represent Jim’s cat, or perhaps “the cat of Jim.”
In most cases in the martial arts you can invert the order of the two words and either make them possessive (with an apostrophe) or put the word “of” between them. This is a gross simplification, but it works in most situations with marital arts phraseology.