Mace, pepper spray not exceeding 118 ml (4 fluid ounces)
Quantity limitations: One self defense spray, not exceeding 118 ml (4 fluid ounces), per passenger in checked baggage only.
Self defense sprays containing more than 2% by mass of tear gas (CS or CN) are prohibited.
The device and ingredients must meet the specifications of a self-defense spray as described in 49 CFR 171.8.
The device must include a positive means to prevent accidental discharge (a recessed button can't be the only safety feature; there must be an addition device such as a cap that flips up).
See the regulation: 49 CFR 175.10(a)(9)
Important note: International aviation regulations (ICAO/IATA) prohibit self-defense sprays in both carry-on and checked baggage.