noun

• The lowermost part of a doorway that one crosses to enter; a sill.
• (by extension) An entrance; the door or gate of a house.
• (by extension) Any end or boundary.
• (figurative) The outset of something; the point of entry, or the beginning of an action.
• (aviation) The start of the landing area of a runway.
• (engineering) The quantitative point at which an action is triggered, especially a lower limit.
• The wage or salary at which income tax becomes due.
• The point where one is mentally or physically vulnerable in response to a provocation or to other nuisances.