A pi filter is a filter with a high slope, which is much higher than that of normal filters. The name pi filter or pi element, also known as a Collins filter, is due to the pi-shaped arrangement of the reactances.
Depending on how the reactances are arranged, it can be a high pass or a low pass.
A pi-shaped low-pass filter is an interconnection of two LC filters: two capacitors with a series-connected coil of double inductance. As a high-pass filter, a pi filter consists of two inductors with a capacitance connected in series.