PPI to DPI

And long ago, so long ago that no one remembers it (except very few, most of whom have given up using it), there was SPI meaning Spots Per Inch (including spots shot on drum or film by laser as mentioned by Brad).

DPI is a measure of an output device's resolution. It came to be also used (incorrectly) over the years to define an image's resolution, but in fact, if you are talking about the resolution of your image or artwork, the CORRECT term is PPI.

Your laser printer creates an image by shooting laser spots onto a toner drum that then transfers a spot of toner (dot) onto your paper; High-resolution Imagesetters would should laser dots onto photographic film, and modern day platesetters would shoot laser dots onto photosensitive plate material, etc, etc. All dots, and measured in DPI.

In the old days, scanners used the term Samples Per Inch (SPI), but that dropped off pretty quickly in favour of PPI.

While we're at it, Halftone dots are measured in Lines Per Inch (LPI).

Abigail,

PPI means resolution in Pixels Per Inch; in Ai it is primarily related to raster effects and/or placed images and/or rasterization in connexion with exporting to raster formats, meaningless in connexion with vector (parts of the) artwork; in Ps it is inherent in the raster image (parts) normally forming the (main parts of the) artwork; in addition it is used in connexion with the final result presented on screen.

DPI means resolution in Dots Per Inch; it is used in connexion with the final result presented in print.

It is preferable/covenient to use the concepts PPI and DPI for diffferent purposes/stages in the work according to their (original) meanings for clarity/avoidance of confusion.

As with other comparable concepts there are some drifts and muddling, such as the use of the native Ps concept canvas drifting into Ai and more or less replacing the native Ai concept work area over about a decade and a half or something like that.

Choosing a resolution of your Ai document/file is only meaningful in the sense choosing a raster effects reolution, because the vector artwork/Artboard/work area/canvas can have no inherent resolution.

 And why do you think you need to do that?

If some asked for file with XX dpi, then they do not know what they're talking about.