This is one of the phenomena that are studied a lot by experts, along with other facts, under many points of view. The one we are interested in is psycholinguistics, that is an interdisciplinary field between neuropsychology, cognitive psychology and linguistics.
This phenomenon where people seem to temporarily forget a certain word, is normal and occurs to everyone at least once in a life. The common expression is tip-of-the-tongue, but the terminology adopted by psycholinguists seems to be simply TOT.
If you want to go more in depth you can read Sparkling at the end of the tongue: The etiology of tip-of-the-tongue phenomenology by Bennett L. Schwartz.