A data structure which grows bigger, or lives longer, than might be expected. Such unexpected memory use can cause a program to require more garbage collections or to run out of heap. Space leaks in functional programs usually result from excessive laziness. For example, the haskell function
sum  = 0
sum (x:xs) = x + sum xs
when applied to a list will build a chain of closures for the additions and only when it reaches the end of the list will it perform the additions and free the storage. Another example is the function
mean l = sum l / length l
The sum function forces the entire list l to be evaluated and built in the heap. None of it can be garbage collected until the length function has consumed it.