A word meaning 'every 150 years.'

The prefix is derived from Latin, semisque. Semis means 'half a unit', while que means 'and'. 'Sesquicentennial' means, literally, 'a centennial and a half.' (For the curious among us, 'centennial' is derived from centum, meaning 'a hundred', and annus, meaning 'a year'.)

Used as a noun, this word means 'the 150th anniversary of an event'.

As an example, the date of this writeup marks the sesquicentennial of the Allied armies, including those of Britain and France, landing in Crimea during the Crimean War.