prevent or hinder a prosecution in order to commit obstruction of justice; it also works the other way around. In other words, if one "corruptly" attempts to cause a prosecution, that too is an obstruction. (The word corruptly is taken from the law/code.)

TITLE 18 > PART I > CHAPTER 73 > § 1503

§ 1503. Influencing or injuring officer or juror generally


(a) Whoever corruptly, or by threats or force, or by any threatening letter or communication, endeavors to influence, intimidate, or impede any grand or petit juror, or officer in or of any court of the United States, or officer who may be serving at any examination or other proceeding before any United States magistrate judge or other committing magistrate, in the discharge of his duty, or injures any such grand or petit juror in his person or property on account of any verdict or indictment assented to by him, or on account of his being or having been such juror, or injures any such officer, magistrate judge, or other committing magistrate in his person or property on account of the performance of his official duties, or 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, shall be punished as provided...

http://www4.law.cornell.edu/uscode/html/uscode18/usc_sec_18_00001503----000-.html