It is when a word represents a sound...i.e..BANG, BOOM, ZAP.
Cat: Meow Clock: tick tack Fighting: Wham!, Pow!
"Click Click .. click click" Like hearing the things lock in place going up the hill.Or "Swosh" going down the hill.
Onomatopoeia is a sound word. For example, bam and boom are not names, but is used to describe sound.
* oak tree * oasis * orange (the fruit) * oatmeal * oar * octagon * object * observation * oboe * over * coat * otter * ostrich * ocelot * octopus * odor * ode * oil * office * onion * omlette * opal * operator * opener * orangutan * otter * oriole * orphan