Some more which I believe have not yet been covered:
Mega: Very rare species. A species whose rarity value is such that the commited twitcher will usually take a sicky within 24 hours to connect with it. Usually several will occur annually on the UK mainland, with more occurring on the far flung islands outposts. Firsts for Britain obviously, also once-in-a-decade species and otherwise often very tricky, stunning or hard to get species.
Purple Patch: Short time period in which an individual or distinct area will rejoice in superlative bird records. Eg a small inland reservoir enjoys 2 Uk Scarcities and a rarity all within the same week. Or you may have an exceptional month for finding your own rarities.
Bogey Bird: A bird which remains elusive, despite the odds. eg not having seen Long eared owl, even after numerous trips to 'supposed' 'dead cert' roost sites, or not recording a common bird like Redshank on local reservoir patch.
Or a seasoned twitcher continually dipping on a 'common' rarity, such as Gyrfalcon, just missing out or being unable to twitch due to other ufortunate events, such as being on holiday abroad or unexpected death in the family. . .
Passage migrant: Bird typically seen only on passage in this country (although may also rarely breed or overwinter) eg the likes of Hoopoe and Bluethroat. 'On passage', or passing through!
Fall: mass grounding of migrants, usually on the coast after certain weather conditions. Typically rain forces migrants down on the first available land after birds have been caught up in bad weather en route from the continent. May include rarities and scarcities, but typically large numbers of 'common' migrants such as Willow Warbler or Robin (date dependant), sometimes even only a single species being involved.