JK> Does anyone know the difference in meanings between
JK> kilesa and uppakkilesa? The dictionary definitions
JK> suggest defilement or impurity for both. What
JK> subtlety of difference does the upa- prefix bring, if
JK> any?
I guess it is something like en-dirt-ment.
The key difference is that 'kilesa' is the term used in the Suttanta
Pitaka, and 'upakkilesa' is used in the Commentaries.
I think that the term 'upakkilesa' is more natural and to the point,
since it is not just impurity, but en-spoil-ment, slime on something
(in most cases on citta).