Original: Circuit Implementation - transistor-level design of basic elements (gates, multiplexers, latches etc) as well as of some larger blocks (ALUs, caches etc) that may be implemented at this level, or even (partly) at the physical level, for performance reasons.
Simplified: Circuit Implementation: Transistor-level design of basic elements (gates, multiplexers, flip-flops, etc) as well as of some larger blocks (ALUs, caches etc) that may be implemented at this level, or even at a lower physical level, for performance reasons.