Glossary as bellow:
Vs
D.C. Spark-over Voltage
The voltage at which the GDT sparks over with slowly increasing d.c. voltage.
V
Impulse Spark-over Voltage
The highest Voltage which appears across the terminals of a GDT in the period between the application of an impulse of given wave-shape and the time when current begins to flow.
Va
Arc Voltage
Voltage drop across the GDT during arc current flow.
Vgl
Glow Voltage
The peak value of the voltage drop across the GDT when a glow-current is flowing, It is sometimes called the glow mode voltage.
8/20 μs
8/20 Current Impulse
Current impulse with a nominal virtual front time of 8 μs and a nominal time to half-value of 20 μs.
1.2/50 μs
1.2/50 Voltage Impulse
Voltage impulse with a nominal virtual front time of 1.2 μs and a nominal time to half-value of 50 μs.
I
Alternating Discharge Current
The r.m.s. value of an approximately sinusoidal alternating current passing through the GDT.
In
Nominal Discharge Current
Crest value of the current through the GDT having a current waveshape of 8/20 μs.
Imax
Maximum Discharge Current
Crest value of a current through the GDT having an 8/20 μs waveshape and magnitude according to the manufacturers specification. Imax is equal to or greater than In.