Analysis of electronic wavefunction generated by quantum chemistry codes has crucial role in exploring nature of electronic structure and providing valuable information of chemical interest. A file containing wavefunction information is inevitably needed as a communicator between wavefunction analysis codes and quantum chemistry programs. There have been many available formats designed for recording wavefunction, such as .fch, .molden, .wfn, .wfx and so on, however they all have different flaws and thus bringing evident inconvenience for development of new wavefunction analysis codes. To overcome this problem, in this article we define a new format "mwfn" (acronym of "Multiwfn wavefunction file") for electronic wavefunction storage and exchange purposes. This format is strict, concise, extensible, and able to provide all kinds of information for common wavefunction analyses. Since the "mwfn" format fully eliminates all shortcomings of existing formats, we expect it will become a standard for recording wavefunction in the field of wavefunction analysis and quantum chemistry.