Hi Themi. ;)
What I meant is the term "cryptic species", first coined in 1940, as cougr pointed out, is probably more relevant today (due to advanced molecular science and DNA techniques).
I also agree with cougr (and everyone else) that "cryptic" means "hidden" of course, whether we are talking about a cryptic species or a cryptic animal ( using "crypsis" or camouflage).
On cougr's point that -
prior to the advent of molecular genetic analysis, when the differentiation of morphologically similar yet distinct species was based on behavioural traits, morphological discontinuities, cytological characteristics etc,,
To my way of thinking, this "differentiation", even if it's based on these early pre DNA/ molecular genetic techniques mentioned by cougr, above, still relies on a form of "decryption or decoding"; in that there's small clues in the unique way an animal behaves, its unique cell biology/cytological characteristics, slightly different spots and markings, etc; and all these tiny clues (even if they are not at the DNA level) are still, in a way, "cyphers", "encryptions" or "markers" which were "decipherable" by the early techniques, back then.
Gene sequences are just the present day "markers" and we have advanced molecular science to help decrypt the DNA.