Terms, Definitions and characteristics

Basic definition
The filter is the passive four-terminal network, transmitting (or retenting) the defined frequency band.

Course of attenuating characteristic of the filter
Other terms and definitions

Title	Symbol	Definition
Nominal frequency	fo [MHz]	Indicates the nominal center frequency of the band-pass or the nominal stop frequency of a band-stop filter
Poles	[--]	The number of poles of the transfer function
Nominal bandwidth (Pass band)	B1 [kHz]	Minimal bandwidth of the pass-band at the attenuation A1 [dB]
Stop-bandwidth	B2, B3 [kHz]	Separation of frequencies between the relative attenuation of filter shall be equal to, or greater than, a specified value
Insertion loss	L [dB]	The logarithmic ratio of the power delivered to the load impedance before insertion of the filter to the power delivered to the load impedance after insertion of the filter
Pass-band ripple	R [dB]	Difference between the maximum value and the minimum value of attenuation within the pass-band B1 [kHz] of the filter
Alternate attenuation	A3 [dB]	Attenuation in a frequency range far outside of the pass band
Spurious response	A4 [dB]	Minimum attenuation caused by extraordinary response in the stop-band. Spurious response usually appears at higher frequency than the center frequency
Group delay distortion	Δt [ µs ]	The difference between the maximum and minimum group delay within the pass-band (B) unless otherwise specified
Terminating impedance	Zt Rt // Ct [ Ω // pF ]	Either of the impedances presented to the filter by the source or by the load

Reference
For more information about the crystal filters can be obtained from the following technical standards: