What is the name for playing one or more notes at same time on violin?

The term you are looking for is Double Stops. Double stops are a technique used in violin playing in which two strings are played simultaneously. This produces a richer, fuller sound than playing a single string. Double stops can be used to create chords, melodies, and harmonies. They are an essential part of the violin repertoire, and are used in many different genres of music, from classical to folk to jazz.