Clock tree
The network of oscillators, multipliers and dividers that decides what frequency each part of a microcontroller runs at — and whether it gets a clock at all.
At the root sit one or more sources: an internal RC oscillator that starts instantly but drifts with temperature, an external crystal that is accurate but takes milliseconds to settle, and often a 32.768 kHz crystal for the real-time clock. A PLL multiplies the frequency up, and prescalers divide it down for the various buses. Each peripheral hangs at the end of a branch behind its own gate.
That gate is behind one of firmware's most common bugs: you configure a UART in loving detail and nothing happens, because its clock was never switched on. The tree governs more than speed. A faster core clock needs more flash wait states, baud rates are derived from the bus clock, and clock gating is the first tool you reach for when power has to come down. Vendors ship graphical tools for drawing the tree, which tells you something about how much fun it is by hand.