BIL is basic insulation level as per you or in other words it
would be "basic impulse level" which implies the limit up to which
an insulator could withstand impulse due to lightning strikes.
Impulse is generated on the insulation due to the high voltage
surges and spikes due to lightning strikes. BIL is generally much
higher as compared to power frequency withstand voltage.
In other words BIL is lightning Impulse withstand voltage.
Impulse withstand voltage is the voltage that is generated due to
switching operation and it's much higher than power frequency
withstand, but BIL or the lightning withstand voltage is a the
voltage level that an insulation can withstand during surge or
lightning strikes.