Why is the letter "h" (or "H") used to denote entropy in information theory and, ergodic theory, and physics (and possibly other places)?
Edit: I'm looking for an explanation of the original use of "H". As Ilmari Karonen points out, Shannon got "H" from Boltzmann's H-theorem. So (assuming Boltzmann actually used "H"), the original use is at least as early as that.