What is the definition of a yacht tender?

A yacht tender is any boat that is used to service yachts, such as a motorboat used to ferry passengers from shore to a boat anchored out at the moorage. I know of no specific requirements for such boats beyond those laid down for any boat of that particular size.

A yacht tender is a boat that goes to and from shore when the yacht is anchored out. It is raised on davits to store on the yacht.It is motorized so it has to be registered .

Well, first of all.. which Coastguard? You don't actually say where you are, so it's hard to be specific.... In general though, the "tender" of a yacht is the smaller dinghy used to ferry passengers and goods, or to assist in mooring, cable laying, etc.... essentially, for operating in shallow draft or conditions of restricted space, where the main vessel can't go While a tender is usually registered as "tender to....<name of yacht>", it is still a vessel in its own right, when not solidly attached to its parent, and must comply with local rules on the operation of registration of small boats... In particular this means that, even when using it to weave a random path back from the marina bar, in the middle of the night, you still have to show lights, observe the anti-collision regulations and remember to apologise to all those people whose boats you boarded by mistake, before you found your own. (Another good argument against sleeping with the foredeck hatch open...)

Anything under 16', and must meet all laws of a boat that size

a yacht tender is beneath the sail and starboard.

A person who takes care of the boat while the owners are away.