Burglary is generally the unlawful entry of a building or structure to commit a felony criminal offense (crime), theft, or assault. The definition of burglary varies from state to state (and in the federal system), and some states have different degrees of severity of the burglary offense, depending on the nature of the unlawful entry (type of building or structure), the intended purpose, and the illegal action taken by the defendant once inside the building or structure. In some states, breaking into an automobile, a coin-operated machine, or an ATM (automated teller machine) with the intent to commit a theft may also constitute the crime of burglary.

A defendant generally commits the offense of burglary of a building or habitation (1) by entering a private habitation or any part of a building that was not open to the public, (2) without the consent of the owner, and (3) with the intent to commit a felony, theft, or assault.

A defendant may also commit the offense of burglary of a building or habitation by entering with consent, but remaining hidden with the intent to commit a felony, theft, or assault—or entering with consent and attempting to commit a felony, theft, or assault, or actually committing a felony, theft, or assault.

Some states make a distinction between burglary of a habitation (a place used for overnight accommodations) and burglary of a building that is not a habitation (home). Burglary of a habitation is usually treated as a more serious offense with greater penalties.

In Illinois, burglary is defined under 720 ILCS 5/19-1 as knowingly entering or without authority remaining within a building, house trailer, watercraft, aircraft, motor vehicle, railroad car, or any part thereof, with the intent to commit therein a felony or theft. This definition does not require breaking and entering; unlawful entry or remaining is sufficient. Illinois law distinguishes between burglary and residential burglary (720 ILCS 5/19-3), the latter involving the unlawful entry into someone's dwelling with the intent to commit a felony or theft, which is considered a more serious offense and carries harsher penalties. Additionally, Illinois recognizes possession of burglary tools (720 ILCS 5/19-2) as a separate offense when one possesses any key, tool, instrument, device, or any explosive suitable for breaking into a building, vehicle, or other structure with the intent to commit a felony or theft. The state does not require the actual commission of the intended crime for a burglary charge; the intent to commit a felony or theft is sufficient for the charge to apply.