Glossary

Bit depth:

Bit depth refers to the number of bits used to construct the color/tone of each pixel.  For color, 8-bit is the same as 24-bit as there are 8 bits for each of the three color channels: red, green, and blue (RGB).  16-bit is the same as 48-bit since there are 16 bits per channel.

Black and White:

Bitonal (1 bit) images: Each pixel can be black or white
Grayscale (8 bit) images: Each pixel can be one of 256 shades of gray

Color:

RGB: red, green, blue – the colors of light used by monitors, scanners, projectors, cameras, etc., to create all other colors
CMYK: cyan, magenta, yellow, black – the colors used by printing presses, and many copiers and desktop printers, to create all other colors

Derivative File:

Copies of the master file that have been modified and saved.

Master File:

The highest quality digital image.  Any modifications should be made to a duplicate of the file, e.g., a derivative file. 

Megapixel:

A megapixel is one million pixels.

Pixel:

The smallest unit of information in an image.

Resolution:

Resolution is the density of pixels in a given area.  Digitized images are made up of a fixed grid of pixels. Therefore a 100 pixels-per-inch (PPI) image will have a grid of 10 pixels across and 10 pixels down in every inch; a 400 ppi image will have a grid of 20 pixels across and 20 pixels down in each inch.

Resolution terms:

DPI, PPI, and SPI are often used interchangeably which is incorrect and can be confusing:
PPI:  pixels per inch — describes the digital resolution of an image
DPI:  dots per inch — describes the output resolution of a given printer
SPI:  samples per inch — describes the number of samples per inch a scanner takes of an image
LPI:  lines per inch — relates to printing method and is very different from the other terms