18 U.S.C. § 1503 defines "obstruction of justice" as an act that "corruptly or by threats or force, or by any threatening letter or communication, influences, obstructs, or impedes, or endeavors to influence, obstruct, or impede, the due administration of justice."
Someone obstructs justice when that person has a specific intent to obstruct or interfere with a judicial proceeding. For a person to be convicted of obstructing justice, that person must not only have the specific intent to obstruct the proceeding, but that person must know (1) that a proceeding was actually pending at the time; and (2) there must be a connection between the endeavor to obstruct justice and the proceeding, and the person must have knowledge of this connection.
§ 1503 applies only to federal judicial proceedings. Under 18 U.S.C. § 1505, however, a defendant can be convicted of obstruction of justice by obstructing a pending proceeding before Congress or a federal administrative agency. A pending proceeding could include an informal investigation by an executive agency.