The term extremely large telescope (ELT) has commonly been used to mean a telescope considerably larger than the 10-meter-class telescopes built in the 1990s and 2000s, using 20 meters (or something similar) as a defining minimum aperture. As of 2019, some are funded and under construction but none are complete and the largest operating telescopes still have apertures of on the order of 10 meters (though some can use multiple such units in concert). A typical ELT design is to use a segmented mirror with adaptive optics.
Current plans and construction:
Some past and speculative concepts:
The E-ELT has taken the name the Extremely Large Telescope aka the ELT.