microvolt

The microvolt is where measurement starts to fight physics for the signal. Radio is the trade that lives deepest in it: received signal strength at an antenna terminal is rated in microvolts, and the amateur radio S-meter convention pins S9 at 50 µV into 50 ohms, with each S-unit a factor of two in voltage below that. A GPS signal arrives at well under a tenth of a microvolt, below the receiver's own noise, and is recovered only because its spreading code lets the receiver average it up out of the hiss.

That hiss is this unit's defining companion. Every resistor generates thermal noise of $\sqrt{4kTRB}$ volts RMS, and the numbers land exactly at this scale: a 1 kΩ resistor at room temperature produces about 4 µV RMS over a 1 MHz bandwidth. No amplifier quality removes it, since the noise is in the source itself; the only levers are colder, lower resistance or narrower bandwidth, which is why radio telescope front ends are cryogenic and why precision voltmeters filter hard. At the microvolt level the universe is audibly warm.

On this site the unit mostly serves as an honest input scale: an EEG amplitude or a thermocouple reading can be entered in microvolts wherever a solver asks for a voltage, and the caution above is the practical reason a pair of dissimilar-metal junctions in the measuring loop can manufacture more microvolts than the signal being measured.