Definition
Describes weather or the environment as hot. Used when the air or surroundings feel uncomfortably warm.
Example
今日はとても暑いから、水を飲んでね。
Show translation
It's very hot today, so make sure to drink water.
Etymology
From Old Japanese *atsushi*, related to the root *atsu-* meaning "thick/dense/concentrated," which extended to sensations of heat and pressure. The い-adjective form 暑い specifically denotes ambient heat (weather, room temperature), while the homophone 熱い (also *atsui*) refers to heat of objects you can touch.
Cultural note
Japan's summers are famously hot and humid, especially from late June through August. *Atsui* is one of the most common conversation-openers during this season — saying 「暑いですね」(*Atsui desu ne*, "It's hot, isn't it?") to a neighbor or colleague is a perfectly normal social greeting. Be careful not to confuse 暑い (ambient heat) with 熱い (*atsui*, hot to the touch, e.g. coffee) or 厚い (*atsui*, thick) — all three are homophones written with different kanji, a classic Japanese false-friend trap.