Honorific noun for the act of standing, typically used in formal or respectful situations.
The president's departure is at 2 p.m.
We are waiting for the honored guests to stand.
お立ち is the polite/honorific way to refer to someone's departure, built directly on the noun 立ち.
Noun formed by adding the honorific prefix お to the continuative form (連用形) of the verb 立つ (たつ, 'to stand'). The meaning extends to 'departure' from the idea of standing up to leave.