The green of unripe olives
Olive is the dark yellowish green of green, unripe olives. Many people call it olive green. It has been a color word in English since late Middle English, the form of the language spoken before about 1500. The Wikipedia article on olive as a color has photos of olive trains, cars and uniforms.
One of the 16 first web colors
#808000 is the CSS keyword olive. It was one of the 16 basic colors of early HTML, which the HTML 4.01 standard fixed in 1999. An earlier HTML spec described these 16 as the standard colors that Windows could show on the PC screens of the time. That explains the simple numbers. Red and green are both set to 128 and blue is 0, so olive is just yellow at half strength. Navy and maroon were made the same way from blue and red.
You can see all the old keywords side by side in the CSS color names list.
Army greens and other relatives
Armies made olive tones famous. Olive drab, a browner shade, was first recorded as a color name in 1892, and American soldiers wore versions of it in World War II. The web also has dark olive green as its own keyword. A lighter cousin, olivine, is named after a green mineral.
Olive is a middle tone, so text on it needs care. White text gets a contrast of 4.20 to 1 and black text 5.01 to 1. Only black passes the usual rule for small text.