Filter cutoff frequency is conventionally the 3-dB-down point.

Evidence

The cutoff frequency is a boundary in the filter’s transfer function at which attenuation begins to occur. In many cases, the cutoff frequency is defined as the point that is 3 dB below the maximum gain, the 3-dB down point. — Jennifer Lentz, Psychoacoustics- Perception of Normal…, 425

Categorization

• DEFINE: Defines the technical meaning of cutoff frequency as the 3-dB-down boundary, not where attenuation first begins.