Whether what follows is the "formal" definition of tender, I really don't know. May not be after reading responses of the earlier responders, BUT:
From encounters with a person that has managed a sailing establishment that owned boats outright and which also took on took well maintained and sturdy boats from owners looking for relief from on-going ownership costs in exchange for the extra wear & tear of their "pride-and-joys": It was mentioned that after taking a prospective new fleet boat for a trial sail, it was determined to be too "tender" for addition to the fleet. (Actually the boat was 1980's vintage but in very good condition). At first seemed to sail fine, but when the wind velocity and water wave conditions began to get a bit testy, the boat became a lot more difficult to sail than the typical boat of its size and type. For instance, un-reefed, this prospective boat began noticeable weather helm at say only 10+kts wind. At 15-20 kts, it was rounding up into the wind in a very pronounced manner with the risk of broaching in the hands of an inexperienced helmsman. It was tender -- needed to be sailed with a lot of skill. Another model of boat in the same general size/class/size might not start doing this until say 15kts and 20+ kts respectively. More forgiving.
(p.s this wasn't my boat by-the-way. I like it too much to rent it out. And while it might not be a racing speed demon, I don't think it is too tender either.)
