Extremely Large Telescope (ELT) is a general term for a very large telescope, commonly using 20 meters as a defining minimum Aperture. As of 2017, some are funded and under construction but none complete and the largest current telescopes have apertures of about 10 meters. A typical ELT design is to use a Segmented Mirror with Adaptive Optics.
Current plans and construction:
The E-ELT is sometimes referred to as the Extremely Large Telescope or the ELT.
European Extremely Large Telescope (E-ELT)
Giant Magellan Telescope (GMT)
Thirty Meter Telescope (TMT)