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A virtual observatory (VO) is an online database of astronomical data, e.g., catalogs. The International Virtual Observatory Alliance (IVOA) coordinates efforts, such as offering standardization regarding features and interfaces. NASA uses the term virtual observatory for its NASA Astronomical Virtual Observatories (NAVO) program, which consists of their online astronomical-data archives, MAST, HEASARC, IRSA, and NED.
I believe the term virtual observatory is also used more generally, including other types of websites that provide some of the benefits of using an observatory, e.g., offering the means to view (images of) a portion of the sky.