Ostensible - ostentatious

Don't confuse two adjectives that start with the same letters - osten - and are ultimately derived from the same root. To confuse them is to be guilty of a malapropism. They have very different meanings, and connotations.

• Ostensible means 'apparent', '[something] which seems to be so'. There is a strong denotation of 'but it is not really so'. A woman may be described as having an ostensible reason for a meeting, but her ulterior or real motive may be very different. OED cites 'Spy' in the New Yorker of 14th May 1992 as saying "One wonders if this ostensible pat on the back wasn't actually a sly, bitchy dig."
• Ostentatious describes a person or behaviour. In colloquial terms it means 'showing off': 'demonstrating something [often wealth] in a way calculated to impress', or 'boastful'. It is not a compliment. There is also a noun ostentation, meaning the act, or behaviour, or habit, of showing off, displaying one's material circumstances in a boastful way.

Etymological note: Both words are derived froma single Latin verb, ostendere. This had varied inflections. The past participle was either ostens- or ostent-. The words ostent (two nouns and a verb) and ostentate recorded in OED are all archaic or rare - or both.