The definition and origin of Schnorr groups?
What (and where) is the actual definition of a Schnorr group?
  Where have Schnorr groups first been introduced and who called them Schnorr groups?
  


I don't know who first used the term—the earliest use I can dig up quickly is actually the Wikipedia article, initially drafted in November 2004 by Paul Crowley[1], who also used it a year earlier on sci.crypt[2].  What it means is a subgroup of prime order $q$ of the multiplicative group of the finite field $\mathbb Z/p\mathbb Z$, where $p$ is large enough (say 2048 bits) to resist index calculus on the field and $q$ is large enough (say 256 bits) to resist Pollard's $\rho$ or other generic discrete log attacks on the subgroup.

Cryptosystems defined in terms of Schnorr groups usually do not involve furnishing adversaries with oracles for $h \mapsto h^x$ for secret $x$, so minimizing $(p - 1)/q$ is not important as it is for, e.g., Diffie–Hellman; instead we make $q$ as small as we safely can to improve exponentiation performance.  So usually we don't consider the order-$q$ subgroup of $(\mathbb Z/p\mathbb Z)^\times$ with a safe prime $p = 2q + 1$ to be a Schnorr group.  But you wouldn't be disinvited from parties if you called it a trivial case of a Schnorr group with bad performance.


  
  Where was a procedure to generate Schnorr groups first published?
  


The generation procedure—randomly choose prime $q$ of appropriate size, randomly choose $k$ of appropriate size until $p := kq + 1$ is prime, and randomly choose $h$ until you find a generator $g := h^k \not\equiv 1 \pmod p$—is obvious enough that it's easy to imagine that nobody bothered to write it down in a paper to publish in an academic venue.


  
    results for other signature schemes like Schnorr's are considered as folklore results but have never appeared in the literature
  


What the ‘folklore’ comment means is that everyone tacitly felt they understood the result to be (perhaps intuitively or obviously) true, but nobody bothered to write a proof down or publish it, so there was nothing in the literature to formally cite.