Part of another set.
A is a subset of B when every member of A is a member of B.
Example: B = {1,2,3,4,5}
Then A = {1,2,3} is a subset of B
Other subsets of B include {2,3} or {1,4,5} or {4} etc...
But {1,2,6} is NOT a subset of B as it has 6 (which is not in B)
See: Set