Energy! also named ki!

I guess you probably heard in manga animation or if you practise martial arts the word ki Ki means spirit, energy, what flows. Its a very common kanji in japanese and you mainly see it used as a compound.
You can find it in words like :
: ten ki : weather
: den ki: electricity : byou ki: illness
: ki bun: feelings : gen ki: healthy
You are likely to find the kanji ki written with some more strokes. As the years go by, kanjis tend to be simpler but some words or places try to keep them as their were. You will find it sometimes this way for example in the word REIKI.