I. o‧ver‧night¹ /ˌəʊvəˈnaɪt $ ˌoʊvər-/ adverb
[Word Family: noun: night, midnight, nightie; adverb: overnight, nightly; adjective: nightly, overnight]

1. for or during the night:
Pam’s staying overnight at my house.

2. suddenly or surprisingly quickly:
He became a millionaire overnight.
happen/appear/change overnight
Reputations are not changed overnight.
► Do not talk about ‘an overnight’ because overnight is never a noun. It is either an adverb or an adjective.

[TahlilGaran] Dictionary of Contemporary English ▲

II. o‧ver‧night² /ˈəʊvənaɪt $ ˈoʊvər-/ adjective
[Word Family: noun: night, midnight, nightie; adverb: overnight, nightly; adjective: nightly, overnight]

1. happening during the night or for the night:
an overnight flight to Chicago
overnight accommodation in London

2. happening surprisingly quickly:
The show was an overnight success.
his overnight decision to become a vegetarian

[TahlilGaran] Dictionary of Contemporary English ▲

overnight
adverb
BAD: I recently stayed an overnight at your hotel.
GOOD: I recently stayed overnight at your hotel.
BAD: I am writing to complain about my overnight at your hotel.
GOOD: I am writing to complain about the night I spent at your hotel.

Usage Note:
Overnight is used as an adverb and adjective (NOT as a noun):
'I missed the connecting flight and had to stay at the airport overnight.' 'If we travel overnight, we'll be there in time for breakfast.' 'The overnight coach arrives in London at six in the morning.'

[TahlilGaran] Dictionary of Common Errors ▲

overnight
to send a document or package somewhere for delivery the next day
We decided to overnight the new contract to our supplier.

[TahlilGaran] English Idioms Dictionary ▲

overnight something
to send a document or package somewhere for delivery the next day
We decided to overnight the new contract to our supplier.

[TahlilGaran] English Idioms Dictionary ▲

overnight
adj.
1. From one evening until the next morning.
We could drive from Chicago to Detroit in one day, but it would be more comfortable if we stayed overnight in a motel.
2. Rapidly.
When Tom won the lottery he became a rich man overnight.

[TahlilGaran] English Idioms Dictionary ▲