A preposition used on its own, i.e. without a following complement. Examples are the italicized words in 'I walked away', 'I dropped out', 'I looked in for a minute', 'I looked up the phone number', 'I looked it up'. Some prepositions are only used intransitively, e.g. 'ashore', 'downstairs', 'here'. Formerly or traditionally classed as adverbs.
Actually the analogy with intransitive verbs means we should say a preposition is intransitive when it lacks an object (a noun phrase that typically refers to a distinct entity). A preposition with an object is, for example, in the kitchen. Prepositions without objects can have various kinds of complement: