There are multiple definitions:

1. face to face(English)

   • immediate feedback
   • OSCE
   • Education
   • https://doi.org…909-021-02585-z

2. filter type feature [selection](English)

   • Filtering (of signals), Medical imaging
   • https://doi.org…859-022-05059-1

3. frequency transfer function(English)

   • a formalism for phase-shifting algorithms
   • OLSE, PSA, PSE, SNR, SPRIT, AWGN, IRLS
   • Signal analysis, Statistics
   • https://doi.org….1364/AO.393153