list of names
1 [countable] an official list or record of names, items, etc; a book that contains such a lista parish register (= of births, marriages and deaths)to be on the electoral register/register of votersCould you sign the hotel register please, sir? (British English) The teacher called the register (= checked who was present at school).
of voice/instrument
2 [countable] the range, or part of a range, of a human voice or a musical instrumentin the upper/middle/lower register
of writing/speech
3 [countable, uncountable] (linguistics) the level and style of a piece of writing or speech, that is usually appropriate to the situation that it is used inThe essay suddenly switches from a formal to an informal register.
for hot/cold air
4 [countable] (North American English) an opening, with a cover that you can have open or shut, that allows hot or cold air from a heating or cooling system into a room compare vent
ˈredʒɪstə(r) 
ˈredʒɪstər 