Burglary at common law is defined as "the breaking and entering of the dwelling place of another at night with the intent to commit a felony therein." Modern law has gotten rid of the "at night" requirement, and has extended the "dwelling place of another" to include commercial buildings and others.
Some of the earliest cases defined "breaking" as using even the slightest force. So, per case law, the courts found that when someone fully opened a window that was already slightly open, it was enough force for "breaking and entering." If you open an unlocked door, that is enough force for "breaking" because you used force to push the door open. However, if the door or window was already wide open, then you have no "breaking."
Though, you must already have the intent to commit a felony when breaking and entering in order for it be burglary. However, modern state statutes might have changed this.