1. A deviation in form, quality, or position of something: Karen bought two variations of the same dress, one was red and sheer, and the other one was blue and was made of cotton.
2. In music, a a technique or method used to produce a variety in the melody, like harmony, orchestration, rhythm,etc.: When Lucy wrote her little piece of music for piano, she decided to use variations in tempo.
3. A related, but a separate thing: In the kitchen, Joe prepared a variation of his recipe for spaghetti sauce, which was fantastic!

In geophysics, the changes in the Earth's magnetic field: Such geomagnetic secular variations occur over hundreds of years and are caused by internal changes in the Earth or a variation of any field or parameter which occurs over hundreds of years.

Any change that happens to the geomagnetic field: A geomagnetic variation can be temporal or short-time changes in the geomagnetic field, both long-term (secular) and short-term (transient).