A law in the United States which banned birth control. Named for Anthony Comstock, an activist for morality by his definition of the term.

When it was passed, the Comstock Act of 1873 was listed under 17 Stat. 598. It's now part of 18 USC 1462 (that's Title 18, Section 1462 of the US Code), which reads:

Whoever brings into the United States, or any place subject to the jurisdiction thereof, or knowingly uses any express company or other common carrier or interactive computer service [ ... ]

Shall be fined under this title or imprisoned not more than five years, or both, for the first such offense and shall be fined under this title or imprisoned not more than ten years, or both, for each such offense thereafter.

The phrase "article ... intended for any indecent or immoral use" was understood to include all artificial forms of birth control, including condoms and cervical caps.

The law was used to prosecute Margaret Sanger and other prominent feminists, and the condom industry in general.

The Comstock Act is still on the books. However, the Supreme Court has struck down many of its provisions. Due to the 1996 Communications Decency Act, however, there is still some concern that it could be used to try to shut down websites like those of Blowfish and Good Vibrations.

The Comstock Act was officially called "An Act for the Suppression of Trade in, and Circulation of, Obscene Literature and Articles of Immoral Use".

"United States v. One Package of Japanese Pessaries", http://womhist.binghamton.edu/mwd/doc25.htm
"US Code: Title 18, Section 1462", http://www4.law.cornell.edu/uscode/18/1462.html