break open

An unannounced intrusion into a dwelling—what §3109 basically proscribes—is no less an unannounced intrusion whether officers break down a door, force open a chain lock on a partially open door, open a locked door by use of a passkey, or open a closed but unlocked door; the protection afforded by §3109 must be governed by something more than the fortuitous circumstance of an unlocked door

Defined by the Supreme Court in - Sabbath v. United States, 391 U.S. 585 (1968).