In
debate-speak (policy/
CX debate), a
permutation (or a
perm in
speed debates) is the option to do
plan and
counterplan simulataneously.
Because counterplans are put forth in an attempt to show a plan that better encompasses the topic and retains higher solvency, a perm (if it works) is the easiest aff stratagey to get rid of the threat of losing to counterplan.
Essentially, if your opponents overlooked the possiblity to performing plan and counterplan and the aff can convince the judge(s) that plan and CP could be done at the same time, the CP threat to loss for aff disappears.