A lookout or look-out is a person in charge of the observation of hazards. The term originally comes from a naval background, where lookouts would watch for other ships, land, and various dangers. The term has now passed into wider parlance.
By analogy, the term "lookout" is also used to describe a person who accompanies criminals during the commission of a crime, and warns them of the impending approach of hazards: that is, police or eyewitnesses. Although lookouts typically do not actually participate in the crime, they can nonetheless be charged with aiding and abetting or with conspiracy, or as accomplices.
A lookout may be used when performing engineering works on an operational railway. They will be responsible for ensuring that all staff are cleared of the track in advance of an approaching train.