Although I'm no expert I've always associated the word "trunnion" with the timber carriage on wheels that carries the cannon, referred to by jeffp as the "cannon truck". But, when dealing with theodolites and other telescope-type surveying instruments, the trunnion is the pair of horizontal 'posts' the theodolite's telescope uses as axles. The equivalent on a cannon would be the pair of horizontal posts (cast with the barrel) mounting the cannon in the timber carriage and used as pivots when sighting/aiming the cannon. I suspect different navies may use different terminology.

Either way, it's more likely to remove one's legs than the truck on any mast.

Cheers, Rowan