TITLE: Seeking reference for the enumerative "mass formula" concept
QUESTION [8 upvotes]: I am teaching a combinatorics class in which I introduced the notion of a "mass formula".  My terminology is inspired by the Smith–Minkowski–Siegel mass formula for the total mass of positive-definite quadratic forms of a given size and genus.  That famous mass formula is much too fancy of an example for my class.  All that I really do is define the concept of the "mass" of a combinatorial object to be $1/|G|$ if $G$ is its automorphism group, and then argue that it can be easier to find the total mass of a collection of objects than to count them straight (using Polya counting theory).  For example, the total mass of unlabeled trees of order $n$ is $n^{n-2}/n!$, because there are $n^{n-2}$ labeled trees.
So I have two questions for which a quick answer (i.e. sooner than two weeks) would be most convenient:

Is "mass formula" a standard name for this concept?  Is there a standard name?
Can someone suggest a free on-line reference, comparable to a Wikipedia page or a little longer?  The class textbook doesn't have a discussion.

REPLY [3 votes]: This doesn't qualify as a free reference, but "Graphs on surfaces and their applications" by Lando and Zvonkin has some nice examples. On p.46, after stating a theorem enumerating trees with a given "passport", the authors remark:
 We will often encounter enumerative formulas where the objects are not counted one by one but a weight us assigned ti each object, and this weight is equal to 1/|Aut|, where the denominator means the order of the automorphism group of the object. Formulas of this kind are often called mass-formulas. (Footnote: The first mass-formula was proposed by H.J.S. Smith in 1867. Mass-formulas are also called Siegel–Minkowski formulas.)