VLBI, or Very Long Baseline Interferometry, is a technique used in radio astronomy that allows for the observation of celestial objects with extremely high resolution. VLBI uses multiple radio telescopes located across long distances to create a virtual telescope with a diameter equivalent to the distance between the telescopes. By combining the signals from these telescopes, VLBI can achieve resolutions far beyond what is possible with a single telescope. This technique is used to study a wide range of astronomical phenomena, including black holes, quasars, and pulsars. VLBI has also been used for geodesy and studying Earth's rotation.