Halfway between red and blue
This is the purple of the CSS keyword purple, one of the 16 basic colors of HTML 4. Its code, #800080, sets red and blue to 128 and leaves out green. That puts its hue exactly halfway between red and blue. Its lightness is also halfway between white and black. The Wikipedia article on purple shades suggests this is likely why it was chosen.
A clash with X11
The X11 color names of 1987 also define a purple, but it is #A020F0, a brighter and bluer color. CSS kept the HTML value, so the two lists disagree. This is one of only a few names where web colors and X11 colors clash. On this site, Purple is the X11 version.
An old name for #800080 is patriarch, first recorded in 1925. The X11 purple has its own old name, veronica. White text on HTML purple reaches 9.42 to 1. For the full list of keywords, see the CSS color names page.

