also: みたざる
expression
less than; not reaching; below
Used in the pattern 〜に満たない to indicate that a quantity, amount, or degree falls short of a certain standard or number. Often appears in formal or written contexts.
参加者は100人に満たなかった。
The number of participants was less than 100.
彼の収入は平均に満たない。
His income is below average.
未満 is a more formal, technical term for 'less than' used in legal or mathematical contexts, whereas 満たない is a verbal expression that can be used more broadly.
From the negative form of the verb 満つ (みつ, 'to be full, to reach a certain amount'), which is archaic in modern Japanese. The expression 満たない is now a fixed phrase meaning 'less than' or 'not reaching'.