boundary vs threshold

boundary and threshold both are nouns.

Word	Noun	Adjective	Verb	Adverb
boundary	Yes	No	No	No
threshold	Yes	No	No	No

As nouns, threshold is a hyponym of boundary; that is, threshold is a word with a more specific, narrower meaning than boundary:

boundary (noun)	threshold (noun)
the greatest possible degree of something	the sill of a door; a horizontal piece of wood or stone that forms the bottom of a doorway and offers support when passing through a doorway
the line or plane indicating the limit or extent of something	the entrance (the space in a wall) through which you enter or leave a room or building; the space that a door can close
a line determining the limits of an area	the smallest detectable sensation
	a region marking a boundary
	the starting point for a new state or experience