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Decoupling capacitor, this name is used for those capacitors which suitably decouple the power lines and pins of the respective ICs. Whereas a bypass capacitor or a bulk capacitor is mainly for restricting the noise from entering into the pcb system. Usually bypass capacitors are used near the power supplies and the power supply pins of the connectors i.e. mainly at the power entry points of the pcb system. Any glitch or noise appearing on the power line is immediately bypassed into the chassis ground and thus prevented from entering into the system. Hence the name bypass capacitor.
Coming to decoupling capacitors, they are usually polarized and mainly act as charge buckets, thus, maintaining the potential near the respective power pins of the components and prevent the potential from dropping down below the supply threshold whenever the component(s) switches at considerable speeds or whenever there is simultaneous switching happening on the board, which calls in for extra demand of power.
Ultimately, its the term bypass cap is used for those capacitors which are used for filtering out the noise and preventing them from entering into the system and decap is used for those capacitors which act as charge buckets...bottom line is..they are all capacitors .