A scalar is simply a single atomic number -- that is not an array.
Atomic values form the cells of arrays but they may also stand by themselves as values, e.g. the result of evaluating 1+2 is the atomic value 3, which is not an array. The Title attribute of any object is an atomic text value.
Because an atom can be seen as a zero-dimensional array, an atoms is functionally equivalent to an array with no indexes. In standard usage the term atom or atomic value is used to distinguish a simple value from an array or list (one-dimensional array).