Criminal. And can be civil too.

Whether it’s by crossing the U.S. border with a "coyote" or buying a fake U.S. passport, a foreign national who enters the U.S. illegally can be both convicted of a crime and held responsible for a civil violation under the U.S. immigration laws.

Attempting to enter or obtain entry to the United States by a willfully false or misleading representation or willful concealment of a material fact (which might include, for example, lying on a visa application or buying a false green card or other entry document).

For the first improper entry offense, the person can be fined (as a criminal penalty), or imprisoned for up to six months, or both.  For a subsequent offense, the person can be fined or imprisoned for up to two years, or both. (See 8 U.S.C. Section 1325, I.N.A. Section 275.)

http://www.alllaw.com/articles/nolo/us-immigration/crime-enter-illegally.html
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Then there is Civil: 8 U.S.C.  Section 1325, I.N.A. Section 275