The Free On-line Dictionary of Computing (19 January 2023):

Distinguished Encoding Rules
DER

    (DER) An X.690 encoding format (or
   transfer syntax) for data structures described by ASN.1
   that specifies exactly one way to encode a value thus ensuring
   a unique, canonical, serialised representation.  DER is a
   restricted variant of BER.  For example, DER has exactly one
   way to encode a Boolean value.  DER is used in
   cryptography, e.g. for digital certificates such as
   X.509.

   (2016-05-05)