Federal Rules of Civil Procedure, Rule 11 (b):
By presenting to the court (whether by signing, filing, submitting, or later advocating) a pleading, written motion, or other paper, an attorney or unrepresented party is certifying that to the best of the person’s knowledge, information, and belief, formed after an inquiry reasonable under the circumstances,–
- it is not being presented for any improper purpose, such as to harass or to cause unnecessary delay or needless increase in the cost of litigation;
- the claims, defenses, and other legal contentions therein are warranted by existing law or by a nonfrivolous argument for the extension, modification, or reversal of existing law or the establishment of new law;
- the allegations and other factual contentions have evidentiary support or, if specifically so identified, are likely to have evidentiary support after a reasonable opportunity for further investigation or discovery; and
- the denials of factual contentions are warranted on the evidence or, if specifically so identified, are reasonably based on a lack of information or belief.
Wikipedia has good coverage of this and related topics.