The corrective optics system installed on Hubble in 1993 was called COSTAR.
COSTAR stands for Corrective Optics Space Telescope Axial Replacement. It was created after engineers discovered that Hubble's primary mirror had been ground to the wrong shape, producing images with spherical aberration. The flaw was tiny, but it significantly spread incoming light and made early observations blurrier than expected.
Astronauts installed COSTAR during Servicing Mission 1 in December 1993. The device used corrective mirrors to compensate for the primary-mirror error in several instruments, while the replacement Wide Field and Planetary Camera 2 included its own built-in correction.
COSTAR was not a general software fix, a replacement primary mirror, or a nickname such as “Fixer.” As newer instruments with internal optical correction were installed, COSTAR became unnecessary and was removed in 2009 when the Cosmic Origins Spectrograph was fitted. Its story remains one of the most famous examples of in-space engineering repair.