lux

The word 'lux' entered English directly from Latin in the late 19th century as a scientific term in physics. Its Latin progenitor, 'lux' (genitive 'lucis'), meant 'light' and is derived from the Proto-Indo-European root 'leuk-', which signifies brightness or light. This ancient root is the source of a vast array of words related to light across Indo-European languages, such as 'light' itself, 'lucid', 'luminous', and 'lunar'. The logic of its adoption into modern English is straightforward: it retains its core meaning of 'light' but is precisely defined as a standardized unit for measuring illumination, thus moving from a general concept to a specific scientific quantity.

The word 'lux' is a simplex loanword from Latin. It is not productively segmented in modern English. In Latin, 'lux' is a noun meaning 'light'.