- Megapixel means "one million pixels"
- A pixel is a coloured dot that goes to make up an image. You need a lot of pixels to make up an image, arranged in rows and columns
- You are already familiar with pixels just from your PC monitor. Your screen definition is probably something like 1600x1200 i.e. 1600 pixels across and 1200 pixels high.
- The image produced by a T5i is 5184 pixels across by 3456 high. 5184x3456 = 17,915,904 pixels = about 18 million.

The size of the sensor does not determine the number of pixels. As you suggest, to produce a large number of pixels on a small sensor then you have to squish the photosites (sensor that generates the pixel) and make them smaller. This has a number of drawbacks:

- the photosite receives less light therefore it is less sensitive
- the photosite cannot record as many levels of light therefore it gives an image that is less subtle and more likely to burn out highlights
- it is more prone to optical diffraction that softens the image