The Collaborative International Dictionary of English v.0.48:

Lossless \Loss"less\, a.
   Free from loss. [Obs.] --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

lossless
    adj 1: characterized by or causing no dissipation of energy
           [ant: lossy]

The Free On-line Dictionary of Computing (30 December 2018):

lossless

    A term describing a data
   compression algorithm which retains all the information in
   the data, allowing it to be recovered perfectly by
   decompression.

   Unix compress and GNU gzip perform lossless
   compression.

   Opposite: lossy.

   (1995-03-29)