A lacuna is the untranslatability of a word from a given language to another, or the accidental absence of a word that is allowed by that languages grammatical rules. In the case of accidental absence, you can turn some verbs in English (recite, arrive, etc.) into nouns by dropping the “e” and adding “al” (recital, arrival). However, describe does not have it’s noun form (describal). Similarly, in English, there is no gender neutral form of “uncle” or “aunt”, and there are no male or female forms of “cousin”. In the case of untranslatability a concept that has a word in one language might not have a word in another. For example, English has no word for someone who is not a virgin (specifically no noun), Romanian has no word for shallow (as in shallow water) and Welsh has no word for 11 (really it’s something like tenty one, but that’s two words so it doesn’t count). Untranslatability is overcome by borrowing words from other languages, creating new words, or translating into a phrase in order to convey that missing concept.