The "Tip of the Tongue"
phenomenon is often referred to as a feature of
Retrieval Failure as a theory of forgetting. A good example of this is if you were to see an old film on TV and recognize the
lead actor, you are certain you know who he/she is but you just can't call their name to mind. This occurs when you have stored the information in your
long-term memory but can't seem to access or find the correct route to it, as opposed to it being lost as in
Decay Theory.
Brown (1991) reviewed 25 years of research into this "Tip of the Tongue" phenomenon and reported that people can generally recall the first letter of the target name or word between 50% and 70% of the time. They also seemed fairly accurate at identifying the number of syllables in the word.
Another point of interest regarding "Tip of the Tongue" phenomenon is the effect of 'interlopers'. These are words that sound similar to the word you are looking for but have no semantic link. An example of this is a man doing a crossword and coming across the clue "South American beaver-like rodent", stumped by this he asks his friend if he knows. His friend replies and says, "I think it sounds like 'coyote'", but will this help or hinder them in finding the correct word, 'coypu'? Perfect and Hanley (1992) investigated this but did not find a clear answer and suggest that it depends on factors such as the distinctiveness of the target word and its similarity to the interloper.
In layman’s terms, it's just where you're missing a link between where the information is stored (your long-term memory) and where it is needed (your short-term memory) at a specific moment. This is not to say that it is irreversible.
Reference sources: My knowledge and "Psychology for AS Level" by Mike Cardwell.