transmission

This is the word mechanics and truckers use for the hardware between engine and wheels. Any combustion engine needs one, because the ratio of engine speed to wheel speed must change as the vehicle speeds up. Gearbox is often used for the whole unit, though strictly it names only the gears; the transmission also includes the clutch or torque converter. Here the word is concrete and freely countable.

The broad sense that the specialized ones narrow down. English borrowed it from Latin transmissio, the noun built on the verb transmittere. Grammatically it works two ways. Uncountable, it names the process (the transmission of power over long distances), usually followed by of and what is sent. Countable, it names a single occasion (two transmissions a day).

The medical sense names the process by which an infection moves on, as distinct from the disease as a thing sent. The route is usually named in front of the noun: contact, airborne or vector-borne transmission. Strictly, the sense covers only spread between hosts of the same species, such as from person to person or within one animal population, not a jump from one species to another.

Neuroscientists use the word for a crossing, not a journey. The impulse travels along the nerve, but transmission is what happens at the gap between cells. Typically one side releases neurotransmitters and receptors on the other side bind them. In this field it most often appears as synaptic transmission, and it is the step that certain toxins and drugs interfere with.

A term of art in civil law, quite unlike the everyday act of sending. It protects an entitlement that has vested but has not yet been used. If an heir or legatee dies before accepting or enjoying an inheritance, the right does not die with them; it passes on to their own successors. It is rarely met outside legal writing, where it often appears as the rule of transmission.