Distance education is defined as education where the instructor and student are separated by space and/or time. Early forms of distance education in the late 19th century used correspondence courses and home study. Instructional radio was used between 1910-1940 but failed to attract large audiences. Instructional television grew in popularity in the mid-20th century. In the late 1970s and 1980s, cable and satellite television were adopted for distance education. The internet and digital technologies became widely used for online distance learning in the 1990s. By 2005 over 3 million US students took online courses and by 2006 over 96% of large US universities offered online courses.