Just to be pedantic...
It used to be 5 watts DC input power to the plate of the final RF power amplifier in the USA. It was not easy for the average CBer to perform this measurement, so the rule was changed to carrier output power. This was set at 4 watts unmodulated carrier and of course 12 watts peak envelope power.
This applies to Class D CB radio.
There are other radio bands available for license free two-way radio.
MURS (multi use radio system) a series of VHF frequencies in the 151 MHz range.
The FCC defines MURS as a private, two-way, short-distance voice or data communications service for personal or business activities of the general public. They are allowed 2 watts of RF output and the highest point of any MURS antenna must not be more than 18.3 meters (60 feet) above the ground or 6.10 meters (20.0 feet) above the highest point of the structure to which it is mounted, whichever is higher.
That is external antennas may be used. The radios are sometimes called color dot radios, because each frequency or channel is represented by a color.
Then there is the FRS (family radio service) in the UHF band. These are the small handheld radios we see for sale and advertised.
These are handheld radios that are limited to a maximum output power of 500 milliwatts or half a watt. The antennas are fixed and not removable. External antennas are not allowed.
Then there are the UHF GMRS radios. Handheld, mobile and base stations are allowed. The last time I checked the maximum power output was 5 watts ERP (effective radiated power). That is the output of the transmitter times the gain of the antenna, minus any losses. The last time I checked these required a license and the payment of a fee. The license term was 5 years.
Some of the small hand held UHF radios that we see for sale are both FRS and GMRS capable and although there may be instructions that a license is needed to use the GMRS channels, very few if any people apply for this license.
These radios are capable of running higher power than the FRS only radios on the GMRS frequencies.
Aye roger D 10-4 good buddy. I am east bound and down, Keep the shiny side up, look out for the chicken coops and do not feed the bears. I'll catch you on the flip flop, I'm gone!