A metaphor - usually a compound expression - used in place of a name or noun, especially in Old Norse and Old English poetry.
The kenning 'battle-sweat' (blood) comes from Beowulf.
Wikipedia has a list of kennings.
Modern kennings include 'ankle biter', 'joy juice', and 'couch potato'...can anyone think of a modern kenning that's even half as cool as the ones in Norse poetry?
Etymology: late 1800s, from Old Norse kenna, to know, to perceive, to name.