The feeling you get when there's a word that you need to say, but you just can't come up with it. This is caused when you have a perfect grasp of the contextual information about the word, but the phonological information is missing. That is, you know exactly what you want to say, but your brain isn't cooperating in actually letting you figure out how to say it.

Previously this problem was thought to be a caused by "blocking", thinking of a word that was fairly close in sound, causing you to only be able to think about that word instead of the one you are actually looking for. A recent study (James and Burke, 2000) has found that this is not the case, and that TOT experiences are actually caused by weak links in the network that connects individual phonemes together into morphemes.

The experiment had two groups, an experimental and a control. Both groups were asked to read aloud a priming list of ten words, and then come up with the single word answer to a question. Each of the answers was a word that is known to commonly cause TOT experiences. The control group received ten words at random that had nothing to do with the answer. The experimental group received five random words, and five words that contained phonemes and phoneme pairs that were similar to the desired answer. Both groups were allowed to answer the desired word or "don't know" or "on the tip of my tongue", so a correspondence could be computed. Probability of finding the word by the experimental group was increased 25 to 50 percent, a statistically significant (drastic, even) jump.

The example I read had the question "What word means to formally renounce a throne?" The experimental priming list contained the words:
Of course, the answer was the word "abdicate".

If you're worried about TOT experiences being a bad thing, or signifying the onset of senility, you shouldn't be. Many older adults find themselves having them more and more often, and stop interacting with others because they perceive it as a problem. Unfortunately, the only way to strengthen the networks is by talking with others and exercising them, so the social withdrawal is doubly bad. Remember that retrieval problems are perfectly natural, and that some time during the conversation a word will probably be said that will trigger memory of the "missing" word.