Mathematical explanation of recursion and lambda (referenced in The Little Schemer)
What Sam said.

Also, it's really well under a page.  If you're familiar with evaluation contexts, you can specify the call-by-value lambda calculus like this:

Terms 

$$M ::= x \mid (M \, M) \mid (\lambda x . M)$$

Values

$$V = (\lambda x . M)$$

Evaluation contexts

$$E ::= [\:] \mid ([\:] M) | (V [\:])$$

The (only) reduction rule:

$$E[((\lambda x . M) V)] \to E[M.\mathrm{subst}(V,x)]$$

where $.\mathrm{subst}$ denotes capture-avoiding substitution.

Again, though, the background knowledge involved in reading this definition is by no means self-evident. There are many free places on the web to read about it. For a tidy and well-typeset presentation, you might also be interested in Felleisen/Flatt/Findler's book, Semantics Engineering with Redex.

Holey Moley! I just googled for it, and got the full PDF online. Well, that won't last...