The MOSFETs in a CMOS logic IC make switching transitions while charging and discharging internal and external load capacitances. The trace impedance during switching can be regarded as an LCR circuit. Since the switching current (i) flows through inductance (L), a spike voltage (=L(di/dt)) appears on the VCC and GND lines of the CMOS logic IC. This noise is called switching noise.
Multiple simultaneously switching outputs draw a large charge/discharge current and therefore cause a large switching noise (called simultaneous switching noise). The following lists the measures for the reduction of switching noise.