Quick and dirty about JIN
Jin ( Ğl ) - is a character composed of three main parts (some may say 2): the right most radical called li that refers to strength or power, the upper left part that refers to moving water and the lower left part that refers to work.
Jin is a character than can have many different meanings in Chinese. It can be a noun, an adjective, a modifier, etc. Moreover, it is my understanding that in different regions of China they will use the character in different ways. More specifically for us martial artists, jin can mean any number of things in any number of arts under any number of teachers. Its not a very specific word. Different systems and different lineages will use it with a slight different emphasis, or in different circumstances.
Trying to stick to the internal martial arts and my bagua in particular -
Jin in its most basic form means the path that power takes (the power from water works through a certain direction (the river) (yeah I'm butchering it - so sue me!)). (I mention above that Jin can be broken down into 2 radicals with the left side combined to mean the water's course, maybe that will make more sense to some here...) Some refer to it as trained power, that, however, might be a value judgement. Some internal martial artists get quite arrogant about things like that.
Anyways... Jin can be modified by certain words to describe a certain type of Jin (here the word jin will often take on a slightly different meaning in English - more similar to a variety of skill or a variety of power) -
Ming Jin - obvious power (literally bright - here meaning visible)
An Jin - hidden power
Ting Jin - the skill of listening
Luo Xuan Jin - spiral JIN
Zheng Ti Jin - whole body JIN
Grammatically different from above, but a commonly heard phrase in American explanations of Internal martial arts is:
Fa Jin - Fa is a verb here that means to send (something) out. Jin is a sort of power directed along a certain path. So you could say that FA JIN just means to send out (a certain type of) power.
Well... this isn't really meant to be a complete explanation of the word in all its uses, just a glimpse into certain types of usage in certain contexts. No time or desire for the former.
I can probably think of exceptions aplenty and other ways to use the word, so I hope some of you native speakers won't get all didactic on me here. Just trying to answer this as best as I can now.