Maybe I am just too old school, but to me a sampler (in hardware terms) could record a sample, edit a sanple and playback a sample. While a number of plug ins mentioned above will do the edit and playback, it will 90% of the time require a stand alone to record a sample whether it is live or as part of a stream of non copyrighted material.
For the last two pre-requisites, here are a lot of these about, I have just bought Iris 2 which will do two out of three, but also have numerous others including simpler which also does the same two out of three.
It is how you record the samples in the first place that is of interest for me who wants to create my own samples as opposed to using library stock.
Not exactly helpful I know, but it helps define the full scope of what is required from a sampler.