According to the Oxford Learner’s Dictionary: preposition is ‘a word or group of words, such as in, from, to, out of and on behalf of, used before a noun or pronoun to show place, position, time or method’.

According to the Cambridge Dictionary: A preposition is defined as “a word that connects a noun, a noun phrase, or a pronoun to another word, esp. to a verb, another noun, or an adjective”.