Modulated continuous wave is defined by the Federal Communications Commission in 47 CFR §97.3(c)(4) as "Tone-modulated international Morse code telegraphy emissions having designators with A, C, D, F, G, H or R as the first symbol; 2 as the second symbol; A or B as the third symbol." See Types of radio emissions for a general explanation of these symbols.
Unlike A1A CW transmissions, A2A MCW morse can clearly be heard on a normal AM radio receiver. It was commonly used by many RDFbeacons to send a morse station identifier on a regular basis.
MCW can be transmitted from any common amateur radio transceiver in AM or FM mode with audio input from an audio tone oscillator or equivalent audio source.  MCW is not allowed in the United States on amateur radio frequencies lower than 50.1 MHz, between 144.0 and 144.1 MHz, or between 219 and 220 MHz, as it is a very inefficient use of radio spectrum. When the amateur radio transceiver is in SSB mode, the resulting modulation is J2A or J2B and therefore not MCW by definition.
F2A MCW morse can be heard on a normal FM radio receiver, and it is commonly used by both commercial and amateur repeater stations for identification. Also, F2A is sometimes used by other types of stations operating under automatic control, such as a telemetry transmitter or a remote base station.