Big Day: a birding event in which a birder or team of birders tries to see as many species of birds as possible within a calendar day.
Big Year: a birding event in which a birder tries to see as many species of birds as possible within a defined area (generally a region or country) within a calendar year.
Dip: to miss seeing a bird which you were looking for.
Fallout: a natural occurrence where migratory birds are forced down by adverse weather in a way that makes them congregate in large numbers. This is generally associated with meteorological and geographical conditions.
LBJ (little brown job): drab birda that are difficult to differentiate and identify.
Lifer: a first-ever sighting of a bird species by an observer; an addition to one's life list.
Lists: a list of all species seen by a particular observer has seen. Often categorised as a life list, county list, year list, etc. Keen birdwatchers may keep several lists and some compete to amass longer lists than their rivals.
Mega: a very rare bird
Patch: a birding location near one's home that a birder visits frequently.
Pelagic: a boat trip designed for birders to find open-ocean (pelagic) species, such as albatrosses.
Plastic: adjective used to describe a bird which has escaped from captivity, rather than a genuinely wild bird.
SOB: Spouse of birder that is not a birdwatcher.
Tick: an addition to a personal list (sometimes qualified as year tick, county tick, etc.). Life tick and lifer are synonymous.
a tart's tick is a relatively common species added to one's list later than might be expected.
an armchair tick is an addition without leaving one's home.
Twitch: the act of traveling a long distance to see a rare bird.
Vagrant: a stray bird far from it's normal range.