clk is the name used for clock signal in a synchronous digital circuit.
First of all, I may correct you that clock may/may not be used in sequential circuits.
Clock is a global signal that synchronises the overall operation of a digital system (called Synchronous Digital System, don't cofuse it with sequential circuit). This golbal synchronization makes the design of digital systems really easy because everything occurs at the rising (or falling) edge of the clock.
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