S

Shock wave:
A compression wave, such as that produced by an explosion. In supersonic flows, shock waves form boundaries over which large pressure differences occur.

S.I.:
S.I. is the abbreviation for "Système Internationale", the International System of units used in science.

Sink:
A sink is a flow disappearing in a point. The flow streams towards the center. The farther away from the sink, the slower the speed of the flow becomes. You can also look at it as the opposite of a source.

Source:
A source is a flow for which the flow originates from a point, giving a flow in the direction away from that point. The velocity decreases with the distance to the origin.

Subsonic Subsonic means "less than the speed of sound."

Supersonic:
Supersonic means "greater than the speed of sound". A flow of which the flow speed is greater than the speed of sound is called supersonic. An object moving with faster than the speed of sound is said to have supersonic velocity. For supersonic speeds, the Mach number is greater than 1.