Simultaneity is the property of two events happening at the same time in a frame of reference. According to Einstein's Theory of Relativity, simultaneity is not an absolute property between events; what is simultaneous in one frame of reference will not necessarily be simultaneous in another. For inertial frames moving at speeds small compared to the speed of light with respect to one another this effect is small and can for practical matters be ignored such that simultaneity can be treated as an absolute property.
Simultaneity appears in Picasso's Still Life with Compote and Glass (1914-15) where we see the circular opening and the profile of the wine glass all at one time. These multiple views show us the wine glass as if we were turning the glass around to see it from different perspectives and remembering each view like a snapshot in our mind. Picasso demonstrates how we know an object conceptually, rather than perceptually.
The theory that we perceive the world in terms of a simultaneous intersection of past and present experience comes from the French philosopher Henri Bergson (1859-1941), whose writings were well known and whose lectures in Paris were very well attended. The Cubists tried to interpret Bergsonian theories through their art by showing different sides of an object or person on one plane. This desire to show "all sides at once" or multiple views became known as "simultaneity."