Question about terminology describing finite multisets.
What is the proper term for the number of items which have a frequency of one? Are they called singletons? Cardinality of singletons? i.e. {a,a,b,c} would be 2 as {b} and {c} only appear once.
Although, I've seen the term cardinality defined both as (i) number of unique elements, and (2) number of items, depending on context; also singleton is defined as a set with only one element, {x}, not necessarily the number of elements in a multiset which have multiplicity of one.
The number of unique entries which appear only once is called the ...
{a,a,b,c} -> 2
{a,a,b,b} -> 0
(There may be another term for the set of such elements?)
Multisets aren't particularly common (probably because they share terminology with regular sets in ways that can make statements unclear), so the terminology isn't very standardized. If $X$ is a multiset, then the quantity you're describing is the number of elements of $X$ of multiplicity $1$. There's no shorter name for it that would be widely understood.