lossless

adj. occurring or operating without loss, free from loss

Lossless data compression is a class of data compression algorithms that allows the exact original data to be reconstructed from the compressed data. This can be contrasted to lossy data compression, which does not allow the exact original data to be reconstructed from the compressed data.Lossless data compression is used in many applications. For example, it is used in the popular ZIP file format and in the Unix tool gzip. It is also often used as a component within lossy data compression technologies.Lossless compression is used when it is important that the original and the decompressed data be identical, or when no assumption can be made on whether certain deviation is uncritical. Typical examples are executable programs and source code. Some image file formats, like PNG or GIF, use only lossless compression, while others like TIFF and MNG may use either lossless or lossy methods.
See more at Wikipedia.org...

Adjective
1. characterized by or causing no dissipation of energy
(antonym) lossy

(a.)
Free from loss.
  

<algorithm, compression> 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)