一同 (yītóng) is a formal adverb meaning 'together', used for joint action by multiple subjects. It is very similar to 一道 (yīdào), another formal adverb for 'together'; both are more common in writing than in casual speech. 同 (tóng) as an adverb is more concise and often appears in fixed expressions, while 一齐 (yīqí) emphasizes simultaneous action, not just joint action.