A Kawamata log terminal pair, abbreviated klt pair, is a normal variety 
with an effective rational divisor 
such that 
is rational Cartier and every log
discrepancy is strictly positive. For a birational model 
, write 
.
The log discrepancy of a prime
divisor 
on that model is 
,
and positivity is required on every higher model. This is stronger than the nonnegative
discrepancy condition defining a log canonical pair.
For example, a smooth variety with zero boundary
is Kawamata log terminal (Kollár and Mori 1998).
These pairs are a basic singularity class in the minimal model program. The same discrepancy definition is used for normal complex analytic spaces when the adjoint is an actual rational Cartier divisor.
