A command meaning 'attention!' or 'stand to attention' used as an order or call to straighten up.
体育の先生が「気をつけ！」と号令をかけた。
The PE teacher called out, 'Attention!'
行進の前に教官が隊列に向かって気をつけをかけた。
Before the march, the instructor ordered the formation to attention.
式典で全員が気をつけの姿勢を取った。
At the ceremony everyone assumed the attention posture.
As a noun: the state or posture of standing at attention; used in military or ceremonial contexts.
将校たちは気をつけの姿勢で並んでいた。
The officers stood in formation at attention.
式典の間、学生たちは気をつけのまま動かなかった。
During the ceremony the students did not move from the attention posture.