Silica, quartz, glass, or oxide, depending on who you ask.
Silicon dioxide (SiO2) crystalizes into either quartz (the most abundant mineral on Earth) or cristobalite, depending on if it's crystal is trigonal or tetragonal, respectively. If impurities (most often sodium carbonate and sometimes lead) are introduced to SiO2 while it is in liquid form and then the mixture is frozen quickly, the crystal structure is disrupted and an amorphous solid is created: that which laymen and experts alike call glass.
SiO2 in almost any form is important because it is usually transparent (or close to being transparent) for the visible spectrum, it is an electrical and thermal insulator, and because it is strong and hard while still having a melting point at a range such that it is easy to manipulate into a usable size and shape.
Silicon dioxide is used extensively in the electronics industry because of the above benefits. It is placed between conductive elements of integrated circuits to keep the wires separate. Chip designers call these layers "oxide" even though "glass" would be a more appropriate term.