Most call it the tip-of-the-tongue syndrome (or phenomenon).
Lethonomia is the inability to recall the right name. Lethologica is the inability to remember the right word. These two words derive from the river Lethe in Hades in Greek mythology. The river was thought to cause oblivion or forgetfulness of the past. Aphasia is an impairment of the language function that is caused by brain damage. Anomia is a deficit in finding words and is the most conspicuous feature of aphasia. Reverse dictionaries and reverse look-up are a great solution to the tip-of-the-tongue syndrome.