Some authors reserve the term "term" for just variables and abstracts. We'll probably just say "term" and "expression" indiscriminately for expressions of any of these three forms.
-<div>
Examples of expressions:
x
(\x (\y x))
(x (\x x))
((\x (x x)) (\x (x x)))
-</div>
The lambda calculus has an associated proof theory. For now, we can regard the
proof theory as having just one rule, called the rule of **beta-reduction** or