A lifeguard is someone who watches over people swimming or playing near water to keep them safe.
You might see lifeguards at beaches and pools, wearing bright orange vests and standing on high chairs called towers. They scan the area for swimmers in trouble and quickly jump into action if they need help. Sometimes lifeguards are also trained to give first aid or CPR if someone gets hurt.
A bodyguard or unit of bodyguards, a guard of someone's (especially a king's) life or person.
