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In digital design, if you take a net or port or pin, there would be a logic cloud driving it, which is called its FANIN cone, and there would be one or more other nets/pins/ports which it will drive. A collection of all those, which a perticualar port/pin/net is driving is called its FANOUT cone.
As an example a D input of a FF in digital design, is point, which would have a FANIN cone, and a Q output of a FF in digital design is a point which will have a FANOUT cone.
A FANIN cone will consist of all signals which D is depending upon
A FANOUT cone will consist of all signals which depend upon Q.
Hope it helps.
If you are asking it in relatation to formality, you may find this site useful: http://www.vlsiip.com/formality