What is the term for being unable to recall a word that is on the tip of your tongue?

Lethologica, or more usually the tip-of-the-tongue state - and the interesting thing is how much you know about a word you cannot produce. People in the state can reliably report the first letter, the number of syllables, and often the stress pattern, at rates far above chance.

That is evidence about how memory is built. It means the meaning of a word and its sound are stored separately and retrieved in two steps, so the link can fail halfway - you have the concept and the shape of the word and not the word. It happens most with proper names, because a name has no meaning to hang on: there is nothing about a person that implies the sound Baker, which is why the same word is easier to recall as a job than as a surname.

The state is close to universal. A survey of 51 languages found that most have an idiom for it and the great majority put it on the tongue, though Korean puts it on the tip of the sparkle and one sign language locates it at the fingertips. It becomes more frequent with age while the rest of vocabulary stays intact, which is one reason it is studied - it isolates retrieval failure from actual loss.