Counter automaton

In computer science, a counter automaton is a pushdown automaton with only two symbols, $A$ and the initial symbol in $\Gamma$, the finite set of stack symbols. This class of automata can recognize a subset of context-free languages, for instance the language

$\{\ a^nb^n : n \in \mathbb{N} \}.$

To accept the above language, let $x$ be a word of the form above. The automaton can use the symbol $A$ to count the number of $a$s in $x$ (writing an $A$ for each $a$ in $x$) and deleting an $A$ for each $b$ in $x$.