一層

layer

one / single / a (article) / as soon as / entire; whole; all; throughout / "one" radical in Chinese characters (Kangxi radical 1) / also pr. [yao1] for greater clarity when spelling out numbers digit by digit

to pile on top of one another / layer; stratum / floor (of a building); story / (math.) sheaf / classifier for layers

to take it up a notch / to bring it up a level

(architecture) basement level 1; floor B1