noun
proper subset
Mathematical term: a subset that is strictly contained in another set, i.e., all elements of A are in B, but B has at least one element not in A.
AがBの真部分集合であるとき、A⊂Bと表す。
When A is a proper subset of B, we write A⊂B.
部分集合 (subset) includes the possibility of equality; 真部分集合 (proper subset) excludes equality.
Compound of 真 (true, proper) + 部分集合 (subset), a direct translation of the English mathematical term 'proper subset'.