Threshold

Threshold 

thresh·old /ˈTHreSHˌ(h)ōld/

Noun

1. a limit or boundary beyond which a particular effect or process is initiated

Every mystery solved brings us to the threshold of a greater one.

– Rachel Carson (1907–1964)

2. a piece of timber or metal that lies beneath a door; an entrance

Up to the threshold of the door there mounted a flight of twenty-seven broad stairs, hewn by some unknown art of the same black stone.

The Two Towers (1954), by J.R.R. Tolkien (1892-1973)