A popular modern artistic style called "pixel art" involves intentionally lowering of the resolution of images to simulate or remind viewers of the low-resolution computers of the 1980s. Uncannily large blocks, or "pixels," are often combined with intentionally constrained color palettes to produce visual motifs that tickle our sense of nostalgia. It's popular in images, video, and games.

The popularity of this style means many people speak and write about it, and they routinely use the word "pixelated" to describe it.

I contend pixelated (adj) or pixelation (v) are the wrong words. The correct words are pixelized (adj) or pixelization (v).

Language is organic, though, and I realize that popular English does not agree with me. Still, allow me to make my argument.

What is Pixelation?

When a real-world scene is converted to a grid of pixels, we can describe this process using many different words, one of which is pixelation. Pixelation means to convert something that is not already represented as pixels into a grid of pixels. That typically means converting an analog image to a digital image.

The most common general verb used when converting something analog such as a visual scene into something digital such as a bitmap image is sampling. Sampling from analog to digital, or digitization, is the process of measuring a sample of the analog source at a specific region and encoding the measured values with a given number of bits of accuracy. An "8-bit sample" would use 8 bits to encode an analog measurement, converting the infinite range of the analog measurement to an integer from 0 to 255 (the range of 8 bits, or 2^8). Digitization is inherently lossy, but today can usually be done using so many bits of accuracy that human beings can't tell that information was lost. And the upside of digitization is that the resulting data can be stored and retransmitted with simple error correction, making signal loss effectively zero.

In the case of an image, the measurements collected while sampling might be the hue, saturation, and lightness (commonly abbreviated "HSL") we detected within the region being sampled.

But "sampling" is a general concept and says nothing about how we defined the regions being sampled.

Using a two-dimensional grid of pixels to enumerate the sample regions of an analog scene, which is what we nearly always do when sampling a visual scene projected by a camera's lens, is pixelation.

A similar word that is used less frequently is rasterization, which refers to raster scanning, the step by step process of operating over (in our case, sampling) a grid from a starting pixel to an ending pixel, such as by scanning a whole row from left to right and then moving on to the next row to do the same. Sometimes named for this process, the resulting data is often itself called "raster" data. Today the common term for a grid-oriented collection of image data is bitmap. But some people refer to bitmap images as raster images.

Other analog sources

Pixelation can also be used accurately when referring to converting a non-bitmap source image such as a vector image into a bitmap image. A vector image can be considered pseudo-analog for the purposes of this discussion because a vector is infinitely smooth, regardless of how far or close it is viewed (that is, regardless of zoom level).

When you use software to convert a vector image to a bitmap (raster) image, you are sampling the image's vectors into a grid of pixels. You are pixelating the vectors—converting the whole image into a grid of pixels.

What is Pixelization?

Pixelization, by contrast, is the intentional lowering of the number of pixels used in an image already composed of pixels. Pixelization is done for style in the case of art, but it's also commonly used to obscure sensitive information in photos or video, as one might similarly use a blur.

When you zoom in on a photograph on a computer, you are pixelizing the photograph. Zoom algorithms usually offer to optionally smooth the resulting edges with a blur to reduce the pixelization effect (when it's not desired).

Since the majority of visual material processed in computers is composed of pixels (the exceptions being vector images and vector fonts), reducing the number of pixels is almost always pixelization—the resulting image is pixelized.

Today's popular artistic trend takes tiny sprites and images composed of very few pixels and zooms in a great deal so they fill our modern high-resolution screens, which is pixelization. Each of the artwork's original pixels is occupying several of the display's physical pixels.

When used stylistically, I refer to the style as pixelized even if one were to, for the sake of argument, individually draw the rectangles necessary to approximate a low-resolution source image rather than actually drawing individual pixels and then zooming in.

Edge cases

What about the result of projecting an image onto a surface within a virtual 3-dimensional scene and then sampling the result? This is surely a gray area since the projected image is itself sometimes a bitmap image, and projections of low-resolution bitmaps onto large surfaces often yield unintentionally pixelized results due to resampling something already composed of pixels as if it were analog.

I personally still use the word pixelized to describe the result; we see rendering artifacts because the source image was composed of pixels already. Had the projected image been analog or vector (and therefore sampled—pixelated—only after being projected onto the virtual surface) it would not be artifacted.

Summary

Pixelation is converting something into pixels. The number of pixels may be large or small. Because the number of pixels is sometimes small, it is now popular to use the word "pixelated" to describe the resulting low-resolution pixelized appearance. But even something composed of a very large number of small pixels is still pixelated. At best, the popular usage of pixelated is imprecise.

Pixelization is intentionally reducing the number of (and thereby increasing the size of) pixels such as for artistic purposes.

A Rule of Thumb

If it's not already pixels and you are converting it to pixels, you are pixelating it. Remember, most things you are working with on a computer are already composed of pixels. So you are rarely doing this at a computer.

If it is already pixels and you are reducing its resolution intentionally to make larger pixels, you are pixelizing it.