What's a pixel?

In principle, it's the smallest possible picture element; but the

translation into practice is not always self-evident (at least to me).

In a digital camera, is a pixel a single sensor; or is it a collection

of 1 red, 1 blue, and 2 green sensors?

In a color inkjet printout, is it one physical dot position (with

cyan, magenta, yellow, and possibly black ink drops overlaid); or is

it a matrix of up to 16x16 dots which collectively can form 256 x 256

x 256 colors? If a pixel is a matrix of dots, does a printer with

variable dot size use a smaller matrix of physical dots to attain the

same color resolution?

On a color monitor, is a pixel 3 physical spots of phosphor glowing

red, green, and blue; or is it the collection of phosphor dots

illuminated by a logical dot of screen resolution (e.g., 1/800 of the

screen width)?