Stinky smell coming from under the floor? Weird light from the garden shed at night? That's an emanation, something, often invisible, that is emitted by something.
Emanation is very similar in meaning to emission, and the two words are often used interchangeably, though there is an important difference; there tends to be something slightly more other-worldy and inexplicable about an emanation, owing in part to the word's theological use in the Christian religion as a means of describing the Holy Spirit. An emission always has a solid base in reality or science, and an emanation may be just plain spooky.