a thief who enters a building with intent to steal
"The security alarm blared at 3 AM after a burglar smashed his way into the jewelry store's back window."
A person who breaks in to premises with the intent of committing theft
"The neighbor called the police after spotting a masked burglar trying to pry open his front door."
To commit burglary; to burgle.
"The homeowner called the police immediately after discovering a burglar had broken into her house last night."