In mathematics, a set is a well-defined collection of distinct objects, considered as an object in its own right.
for example
A = {2, 1, 3}
B = {blue, white, red}
C = {3, 5, 6}
D = {A, C}all the above are sets, A, B, C are regular sets, and D is a set of set. Is there a term do refer to this type of set(D), to distinguish set of other sets (like A, B, C)?
"regular" math objet may not be a precise term, "regular" used here to distinguish a set to from a simple/regular one, such as real numbers that could be an operand of an elementary function.
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$\begingroup$The term "family" is used for sets of sets. This usage is most common when all the elements of the sets in the family are of the same kind (So in your examples $D = \{A,C\} = \{\{1,2,3\},\{3,5,6\}\}$ is a more typical example than say $\{A,B\} = \{\{1,2,3\},\{\mathrm{blue},\mathrm{white},\mathrm{red}\}\}$). Wikipedia:
$\endgroup$ $\begingroup$In set theory and related branches of mathematics, a collection F of subsets of a given set S is called a family of subsets of S, or a family of sets over S. More generally, a collection of any sets whatsoever is called a family of sets.
I was trained to refer to a set of sets as a collection, I suppose so that it would be relatively clear when I was referring to the encompassing set and when I was referring to the sets which were members. A quick trip to Google shows me that this verbiage is somewhere between standard and "not unheard of", so you're welcome to join if you like.
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