A lookout is a person whose job is to watch carefully for danger or for something important. On old sailing ships, a lookout would climb high up the mast and scan the horizon for other vessels, land, or approaching storms. Their keen eyes could spot trouble long before anyone else, giving the crew time to prepare.

The word can also mean a high place used for watching. A fire lookout tower in a forest gives rangers a clear view across miles of trees, helping them spot smoke from wildfires before they spread. Soldiers might establish a lookout on a hilltop to monitor enemy movements.

When you're on the lookout for something, you're watching carefully and staying alert. You might be on the lookout for your friend in a crowded cafeteria, or on the lookout for a sale on the book you want. The phrase keep a lookout means to watch vigilantly: “Keep a lookout for my backpack while I'm gone.”

The phrase be on the lookout (often shortened to BOLO in police work) means to actively search for someone or something. After a storm, a town might ask residents to be on the lookout for downed power lines.