Wiktionary
adv. (non-gloss definition: used to link a semantically weaker word to a semantically stronger word following the adverb, both sharing the same basic meaning, indicating the increased likelihood of the latter)
WordNet
adv. perhaps; indicating possibility of being more remarkable (greater or better or sooner) than; "will yield 10% if not more"; "pretty if not actually beautiful"; "let's meet tonight if not sooner"
Usage examples of "if not".
He judged that there were at least thirty snipers, if not more, and they were all emptying their clips as fast as possible, reloading at top speed, and pouring out ammunition at a rate no one could possibly afford.
Her condition belied her bravado-if not for the support of Flinx and Lauren, she would not have been able to stand.
But hit a man anywhere at all on his body with a bullet from one of them, and hydrostatic shock would knock him out, if not kill him.
The waves were matched in frequency if not intensity by the palpitations of Flinx's stomach.
I think that you must have at the very lowest the belief that Christ was, if not divine, at least the best and very wisest of men.