Low-pressure discharges are discharges made under gas pressures from a few millitorr to a little less than atmospheric. They have the benefit of less power requirement of sustenance of the discharge as volume-recombination rates are lower.
It is easier to achieve uniform discharges at low pressure. Normally, the system is pumped down and only necessary plasma gases are then flown into the plasma chamber. Argon is a typical background gas because it has low ionization potential and therefore is easier to break down and sustain.