Proper complexity function

A proper complexity function is a function f mapping natural numbers to natural numbers such that:

f is nondecreasing;
there exists a k-string Turing machine M such that on any input of length n, M halts after O steps, uses O space, and outputs f consecutive blanks.

If f and g are two proper complexity functions, then f + g, fg, and 2^f are also proper complexity functions.
Similar notions include honest functions, space-constructible functions, and time-constructible functions.