In the equation E=mc², the units used for the constant 'c' (the speed of light in a vacuum) depend on the system of units being used. In the International System of Units (SI), which is the most widely used system, the speed of light is typically expressed in meters per second (m/s).
The energy 'E' in the equation is measured in joules (J), which is the SI unit for energy. The mass 'm' is measured in kilograms (kg), also an SI unit. Therefore, when using SI units, the speed of light 'c' in the equation is expressed in meters per second (m/s).
It's worth noting that when using different systems of units, such as the CGS (centimeter-gram-second) system, the units for 'c' would be different. In the CGS system, the speed of light is typically expressed in centimeters per second (cm/s).