Counter for nights of a stay (e.g., 一泊, 二泊), used to indicate the length of an overnight stay.
来週は温泉に一泊二日の旅行に行きます。
Next week I'm going on a one-night, two-day trip to a hot spring.
出張は三泊の予定だ。
The business trip is scheduled for three nights.
子どもたちは修学旅行で二泊三日だった。
The children had a two-night, three-day school trip.
An archaic or rare noun meaning an overnight stay or lodging (today '宿泊' is more commonly used).
昔の文献では『泊』という語が宿泊を意味することがある。
In old texts, the word '泊' is sometimes used to mean lodging.