To reverse the procedure conducted by the compression software algorithm to return data to its original size and condition.

Decompression is the process of restoring compressed data to its original form. Data decompression is required in almost all cases of compressed data, including lossy and lossless compression. Similar to compression of data, decompression of data is also based on different algorithms. Decompression is considered important, as compressed data needs to be restored back to standard state for usage. Decompression is widely used in data communications, multimedia, audio, video and file transmissions.

Decompression is also known as uncompression.

Compression of data is advantageous as it helps in the reduction of storage space, transmission capacity or usage of resources. Decompression is thus important for compressed data as all compressed data needs to be decompressed. The application needed for decompression largely depends on how the data was compressed in the first place. There are different techniques and algorithms available for decompression of data. In most cases, the software or application needed for data decompression also comes with the application or software used for data compression. For each compression technique, there exists a corresponding decompression technique as well. Certain compressed files, such as those ending in ".exe" and ".sea" are classified as self-extracting files. These files do not need any special application for decompression, as it is initiated automatically once the file is clicked or run. Most decompression software makes use of a decoder, which aids in the data decompression process.

In the case of lossless compression, the original data is obtained without any loss on decompression. This is not the case with decompression for lossy methods, as there could be loss in the original data, although it may not impact the receiver. Data decompression helps in removing the complications added by data compression. Like compression, decompression can also be slow and time consuming at times and errors in transmission are not uncommon during data decompression.