A clear-channel station is a North American AM radio station that has the highest level of protection from interference from other stations, particularly from nighttime skywave signals. This classification exists to ensure the viability of cross-country or cross-continent radio service enforced through a series of treaties and statutory laws. Known as Class A stations since the 1983 adoption of the Regional Agreement for the Medium Frequency Broadcasting Service in Region 2 (Rio Agreement), they are occasionally still referred to by their former classifications of Class I-A (the highest classification), Class I-B (the next highest class), or Class I-N (for stations in Alaska too far away to cause interference to the primary clear-channel stations in the lower 48 states). The term "clear-channel" is used most often in the context of North America and the Caribbean, where the concept originated.

Since 1941, these stations have been required to maintain a transmitter power output of at least 10,000 watts to retain their status. Nearly all such stations in the United States, Canada and The Bahamas broadcast with 50,000 watts, with several clear-channel stations in Mexico going as high as 150,000 watts, and XEW in Mexico City having formerly operated with 250,000 watts for over 80 years before moving the transmitter and reducing to 100,000 watts in 2016. Cuba was originally included in the plan and had several stations given clear-channel status, but stopped participating after the Cuban Revolution of 1959.

60 medium wave frequencies were set aside in 1941 under the North American Regional Broadcasting Agreement (NARBA) for use by usually only one, although in some cases two or three, AM stations, covering a wide nighttime area via skywave propagation. These frequencies were known as the "clear channels", and the stations on them are thus clear-channel stations. NARBA set aside 37 Class I-A frequencies and 27 Class I-B frequencies. The Class I-N stations in Alaska shared those same frequencies. Where only one station was assigned to a clear channel, the treaty provides that it must operate with a nominal power of 50 kilowatts or more. These were for the most part Class I-A. Stations on the other clear channels, with two or more stations, must use between 10 kW and 50 kW, and most often use a directional antenna so as not to interfere with each other. In addition to the frequencies, the treaty also specified the specific locations where stations on Class I-B channels could be built.

Some of the original NARBA signatories, including the United States, Canada, and Mexico, have implemented bilateral agreements that supersede NARBA's terms, eliminating among other things the distinction between the two kinds of clear channel: the original "I-A" and "I-B" classes, and the newer, U.S.-only "I-N" class, which are now all included in class A. Classes "I-A" and "I-B" still mandate a minimum efficiency of 362.10 mV/m/kW at 1 km, whereas Class "I-N" is permitted to use the lower Class B minimum efficiency of 281.63 mV/m/kW at 1 km. There exist exceptions, where a former Class B station was elevated to Class A, yet it maintained its previous antenna system, or made only minor changes thereto.

Clear-channel stations, unlike other AM stations in North America, have protection from interference to their nighttime skywave secondary service area. Other stations are entitled, at most, to protection from nighttime interference in their primary service area—that which is covered by their groundwave signal.

Many stations beyond those listed in the treaty have been assigned to operate on a clear channel (and some had been long before NARBA came into effect in 1941). In most cases, those stations operate during the daytime only, so as not to interfere with the primary stations on those channels. Since the early 1980s, many such stations have been permitted to operate at night with such low power as to be deemed not to interfere; these stations are still considered "daytimers" and are not entitled to any protection from interference with their nighttime signals. Another group of stations, formerly known as class II stations, were licensed to operate on the former "I-B" clear channels with significant power at night, provided that they use directional antenna systems to minimize radiation towards the primary stations.^{[citation needed]}

Daytimers (also known as daytime-only stations) are AM radio stations that are limited to broadcasting during the daytime only, as their signals would interfere with clear-channel and other radio stations at night, when solar radiation is reduced, and medium-frequency radio signals can propagate much farther. Such stations are allowed three manners of operation after sunset; to sign off the air completely until sunrise, reduce power (sometimes dramatically, to only a few watts), or switch to a nighttime-only frequency (such as the Detroit area's WNZK, which broadcasts on 690 during the day, and on 680 at night). Their broadcast class is Class D. A great number of these stations use FM translators to continue their broadcasts overnight, and some also broadcast on the internet and have separate streams that air when the station's over-the-air signal has signed off.

Daytime-only stations first originated in the late 1920s shortly after General Order 40 was imposed. One of the first to do so was WKEN in Kenmore, New York (now WUFO). WKEN proposed the concept to avoid the then-common practice of having to share one frequency between multiple stations; under General Order 40, WKEN would have had to share its frequency with WKBW, and the daytime-only proposal allowed both stations their own frequency. WUFO remains a daytime-only station to the present day, albeit with a 24/7 FM translator introduced in mid-2017.