 Solution

Lazy evaluation is evaluation by need: an expression is deferred until its value is demanded, and its computed value is shared for subsequent demands of the same suspended computation.

Its two key properties here are avoiding evaluation of unused expressions and sharing demanded computations. For example, the first component of (3, undefined) can be obtained without evaluating the second. A suspended expression is represented by a thunk; after evaluation its result can be reused. This supports productive infinite lists: take 5 [0..] demands only a finite prefix. It does not guarantee that a demanded computation terminates.