Singleton representations of $SO(2, d)$? What is meant by "singleton" representations of $SO(2, d)$ and 'small representations' in Witten's paper, Anti de Sitter Space and Holography?
 A: The "singleton" and "doubleton" language comes from the oscillator method of finding group representations.
Given a (non-compact) group $G$ admitting lowest weight representation with maximal compact subgroup of the form $H\times\mathrm{U}(1)$ for some other compact group $H$, the oscillator method is to describe (unitary irreducible) representations by realizing the generators of $G$ as creation and annihilation operators on some Fock space, and have them transform in the (anti-)fundamental representation of $H$.
A singleton representation is one where the generators are realized as the creators and annihilators of a single oscillator, and there are typically two of these (usually the scalar and spinor representations, in a physical language).
A doubleton representation is one where the generators are realized as two sets of oscillators - this obviously extends to the idea of "$n$-ton" representations.
A more thorough discussion is found for example in "Singletons, Doubletons and M-theory" by Murat Gunaydin and Djordje Minic (arXiv:hep-th/9802047v2).
