Boost (turbo) clock
Beginner
A faster clock a chip can use for a short time, or when only a few cores are busy, because it has power and heat to spare.
Novice
A clock rate above the guaranteed base rate that a processor runs at when power, current and temperature allow, for example when only a few cores are busy. Intel’s name is Turbo Boost.
Expert
Opportunistic DVFS within the TDP: with idle cores power-gated, the busy ones get the headroom. Base clock is what all cores sustain at full load; boost is what one or a few can reach.
Explained in Multicore and vector units (Architectures).
See also: DVFS, TDP (thermal design power).