A part taken to stand for the whole. A sample is only as good as the honesty of how it was chosen.
Latin exemplum (something taken out to serve as a model), from eximere (ex- out + emere, to take), via Old French essample; a sample was first a thing removed to stand in for the whole it came from.
A part taken deliberately to stand for the whole — and, true to the word's own root, only as trustworthy as the honesty of how it was pulled out.