A measure of the amount of radio frequency energy (radiation) absorbed by the body when using a radio transmitter device such as a cell phone.
Working closely with federal health and safety agencies, such as the Food and Drug Administration (FDA), the Federal Communications Commission (FCC) has adopted limits for safe exposure to radio frequency (RF) energy. The FCC requires cell phone manufacturers to ensure that their phones comply with these limits. Any cell phone at or below these SAR levels (that is, any phone legally marketed in the U.S.) is a "safe" phone, as measured by these standards.
The FCC limit for human exposure from cell phones is a SAR level of 1.6 watts per kilogram of body tissue (1.6 W/kg). The limit in Europe and some other countries is 2 W/kg.
Phones only operate at the maximum SAR level in extreme situations. Typical RF output is usually a fraction of the maximum, and even less in strong coverage areas (closer to a tower).
Although all phones approved by the FCC are considered safe, the maximum SAR output of a specific phone is usually listed in the user manual (per CTIA guidelines). The information can also be found on the FCC web site. A shortcut to that info is to click on the FCC ID link on the specs page for a phone here on this site.