A pixel is the smallest unit of display.
It is almost universally rectangular in shape. You can think of them as atoms that make up an image, except these atoms are arranged in neat rows and columns like a table!
The more tightly packed the pixels are the more detail you can see in an image.
There isn’t a universally accepted definition, I’m afraid. The amount of detail an image holds determines its resolution. An image with more detail is said to have a higher resolution than an image with lower detail. The problem is, there are many ways one can measure detail.
Some like to use the word ‘fine detail’ to distinguish between regular ‘detail’ – but this just adds to the confusion, because detail also depends on magnification – how far or close you are to a subject.
Horizontal and Vertical Resolution
The number of pixels in the horizontal direction is the horizontal resolution of an image.
The number of pixels in the vertical direction is the vertical resolution of an image.
Resolution is usually written in the form H x V. E.g. 1920×1080, which means there are 1920 pixels in the horizontal direction and 1080 pixels in the vertical direction.
The formula for Resolution is:
Resolution (in pixels) = Horizontal Resolution (in pixels) × Vertical Resolution (in pixels)
Sometimes it is also notated as just 1080p or 1080i, and other times it might be called 2K, 4K, 8K or whatever. You can appreciate why using the long form of H x V is the least confusing.
This last suggestion is going to be a constant theme throughout this site. Engineers use symbols, units, notations and terms in a mathematically precise way. It’s not any more complicated than it has to be, trust me. So please try to resist the urge to truncate, redefine or rename something. You know better!
Total Resolution or Megapixel count
Sometimes, resolution is displayed as total pixel count. E.g. a 1920 x 1080 image has a total pixel count of 1920 x 1080 = 2,073,600, or roughly 2 Megapixels (MP, Mega = million).
Why is talking about resolution in megapixels a bad idea? Imagine an image of resolution 1,000,000 x 2. This is also a 2 MP image but it clearly has very poor vertical resolution. Megapixels by itself don’t mean anything.
Let’s agree to stick to H x V, shall we?
Finally, now that sensors are getting bigger, we can have images of sizes greater than 1,000 Megapixels. This is called a Gigapixel.
1 Megapixel = 1,000 pixels
1 Gigapixel = 1,000 Megapixels