I don't like blanket prohibitions, except for ones canonized in the Help Center, so I'm not going to say tip of the tongue questions are never suitable, just that they are almost never suitable.
A "tip of the tongue" question is just a Single Word Question to which the OP already knows, but temporarily cannot access, the answer. Often, the answer comes unexpectedly hours later.
Sometimes it does not; if the OP is on the verge of madness, and reasonable Googling, which she can summarize, has not worked, then ask the question as a last resort, but convey your desperation.
(Full disclosure; I asked a tip of the tongue question when I was young and inexperienced, and was not closed down.)
This type of question is known across the Stack Exchange network as a “guessing game”. (In this case, guess what word I’m desperately trying to remember). Guessing games often turn out to be a poor fit for what Stack Exchange is trying to do: create a database of expert answers that help future visitors.
One of the founders of Stack Exchange argued in a Stack Overflow blog post that guessing games are commonly:
asked for no practical reason (you should only ask practical, answerable questions based on actual problems that you face)
asked without doing the necessary research (you should document that you’ve invested substantial effort, and give us something concrete that provides us with a reasonable chance of actually guessing the answer)
useless to anybody but the asker (the goal of Stack Exchange is not to construct questions that only help one person)
unfair (do my work for me questions are disproportionate and unfair to the experts on the site; an expert should have at least some confidence that the answer answers the question)
uneducational (no way to learn from the answer about the process of discovery: the expert just happens to be able to guess the correct answer)
The bullet points above are mostly direct quotations from that blog post and this site’s FAQ.
In short, “guessing game questions do not meet our goal of making the Internet better”.
This does not mean all such questions are unfit for Stack Exchange. If the question is practical, answerable, well researched, helpful to future visitors, and requires real expertise rather than guesswork to answer, sure, ask it here.
Generally, if they're too specific, as in, "I forgot this word, it starts with C, and ends with T, had only three letters in it, used for pets, this and that and that and this etc," they're not a good fit for ELU.
But sometimes an interesting one comes along, and we all have fun playing the guessing game, until somebody decides to close it, and meanwhile it would've received a lot of views, upvotes and stars.
So, to your question, I think it's not that well defined.
Abstract: They are generally not considered a good fit for Stack Exchange format generally but we should allow these, because of practical enforcement issues.
Reading Let's Play the Guessing Game by Jeff Atwood. The gist of the article is that identification questions, including single-word-requests which aim to identify a person's sentiment as being the signification of a word, are impractical to answer, unfair to experts and uneducational to anybody, and that this is especially so when they merely constitute lazily vague decriptions of something that even the questioner only just barely remembers.
The way that Jeff Atwood describes how perfectly reasonable answers can be dismissed just because they don't happen to be the right, thing reminds me of this sesame street skit, where Cookie Monster insists that what is in Kermit's mystery box must be a cookie based on the descriptive clues, despite the fact that Kermit knows otherwise and is fully aware of what is in the box.
This makes them poorly suited to our system of peer review. Keep in mind that people will be voting for or against answers based on how closely they match the description, even if they are not strictly the correct answer and a premature vote can give that answer a pretty strong consensus.
Jeff is not alone in his distaste regarding guessing games. It is a common complaint levied against Single Word Requests, and I have noticed that at least three of the other Stack Exchange subdomains have taken steps to restrict if not outright ban identification request type questions, including Anime & Manga, Movies & Television and Arqade.
However, there is presently no network-wide closure reason for it. Too localized might have been considered the right reason in the past but it was too problematic, and too narrow was still considered too restrictive and inspecific for the S.E. format, so what were given instead are the ability make specifically out of scope questions with three prefabricated closure reasons and a write-in closure reason that explains why a question is otherwise outside of scope. We do not currently have any obvious restriction on this within the help center, or a corresponding closure reason.
We could add a restriction to the single-words-requests tag to correspond with its closure reason, and I am tempted to do so as a compromise with the half of the community that detests them to avoid the category from tipping past the brinking point of contention and into a ban for the whole category.
I find a ban to be inadvisable for two reasons:
These Questions Have Definite Answers
The first is that it gives the wrong impression of what the community wants from questions. One of the complaints most commonly levied against them is that they lack definitive answers and that we may be left on a wild goose chase searching for a word that may not even exist, because it describes such a strangely specific sentiment. Closing questions just because they actually have a definitive answer is contrary to the purpose of other closure reasons, and sorting out when we want questions that have a definite answer and when we do not seems like a recipie for disaster that will only confuse everybody involved and lead to uninentional side-effects that are best avoided.
Enforcing Such a Policy Would Be Impractical and Unfair
The other much more important factor is that. There is very little distinction between this type of question and the typical single word request, and it all boils down to the reason the questioner is asking. Any rule against a tip-of-the-tongue question would be unenforceable against anybody who knew it as a result, and could only serve to be an infuriating beginner's trap. Questioners could easily just disguise their question as a broader Single Word Request, simply by omitting the reason for asking and we would be none the wiser.
Any policy that punishes innocently ignorant souls and rewards the knowing criminals, or can be easily defeated by writing a less descriptive, informative and answerable question is not a good policy to endorse.
Therefore, for as long as single-word-requests are allowed, these should be too.
