Cutoff frequency
also known as: Corner frequency · -3 dB point
The frequency where a filter's output power has fallen to half (−3 dB, ~70.7% of voltage). Not where the filter 'stops working' — where its effect becomes significant. For an RC filter: fc = 1/(2πRC).
In practice
Below cutoff a low-pass passes nearly everything; above, it rolls off at 20 dB/decade per pole. Choosing fc is choosing what counts as signal and what counts as noise — put it a decade away from both when you can.