• 警察けいさつ — police
• 警告けいこく — warning
• 警報けいほう — alarm
• 警備けいび — security, guard
• 警戒けいかい — caution, vigilance
• 警官けいかん — police officer
• 巡警じゅんけい — patrolman
• 警視けいし — police inspector

警 is composed of 言 (speech, warning) and 敬 (respect, alertness), suggesting a formal or serious warning or message. It is closely tied to law, security, and civil order.

You'll encounter it commonly in signs, alerts, and names of public agencies like 警察 (police) and 警備会社 (security firm).