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.
