Today's word is "aberration". This isn't the general "deviating from the usual" definition, however. There are two ways to define aberration in a scientific term (I'm sure that there are many more ways, but I'll talk about two).
The first definition is optical aberration. This occurs in two ways. The first way is known as chromatic aberration when the edges of an image are oddly colored. Chromatic aberration occurs when a glass lens refracts light in an uneven way, bending the colors and creating this aberrant effect. The second form of optical aberration is spherical aberration. Spherical aberration can occur with either lenses or mirrors, and it happens when the light rays from the observed object come to focus in slightly different positions as a result of curvature in the observing device.
Chromatic aberration. Source
Spherical aberration compared to no aberration. Source
The second definition is astronomical aberration. Astronomical aberration is the observed displacement in the position of a star due to the Earth's movement around the sun. The star's light appears to come from a spot slightly displaced in the direction of the Earth's movement. This is defined by the equation a = v/c, where v is the orbital velocity of Earth, and c is the speed of light.