Breadth is how broad something is across. The breadth of the river, breadth of experience, a hand's breadth.
The figurative sense is now much the commoner: breadth of knowledge means range rather than depth, and the two are the standard pair for describing what somebody knows. The spelling was reshaped to match length and strength, which is why it looks the way it does. Note broad, the adjective it comes from.
1nounbreadthHow broad something is across.
“Measure the length and breadth of the room before you buy a carpet.”
2nounrangeRange of knowledge or experience.
“The teacher was impressed by the breadth of her reading.”
breadth is a measurement or range.
Middle English bredthe was reshaped from an older brede to match length and strength. It is broad plus -th.
Middle English
bredthe
English, today
breadth
From Middle English bredthe, alteration (due to nouns ending in -th: length, strength, wrength, etc.) of brede "breadth"; see bread. Equivalent to broad + -th. Cognate with Scots bredth (“breadth”), stq Bratte (“breadth”), West Frisian breedte (“breadth”), Dutch breedte (“breadth”), German Low German Breddte, Breddt (“breadth”), German Breite (“breadth”), Danish bredde (“breadth”), Norwegian Bokmål bredde (“breadth”), Swedish bredd (“breadth”).
Etymology from Wiktionary, CC BY-SA
Practised in Numbers & Measurement