It's marketing speak, trying to edge out 1080p/2.1MP. There are a lot of cameras referencing 2.4MP using Sony sensors. I believe it comes from the sensor, like the IMX222LQJ, which list effective pixels as 2.43M. But actual output of the camera is 1080p. The Dahua HDCVI 2.0 cameras are an example.
Ethan is correct. The output, or "effective pixels" of the sensor is 2.4MP but it is usually a native 4:3 aspect ratio which has fallen out of favour compared to 16:9.
The top most and bottom most pixels are perhaps the least valuable from a security perspective, since the typical view has the top and bottom filled with sky and/or ceiling, or carpet and/or grass.
It is usually the same horizontal resolution (1920), but with extra pixels added on the top and bottom. These are the 'missing' pixels that are cropped to display 16:9 (1920x1080).
The default output is usually 1080P since that's what most customers want, but there should be a mode to put the device into the full 2.4MP window.
Depending on the video processor used, there can be a frame rate drop if you include the extra pixels. If 30 fps and 2.4MP is important, be sure to check the specifications. Many of the cheaper video processors only support 30 FPS at 1920x1080P.