The Free On-line Dictionary of Computing (30 December 2018):
A file format using 64 ASCII
characters to encode the six bit binary data values 0-63.
To convert data to base 64, the first byte is placed in the
most significant eight bits of a 24-bit buffer, the next in
the middle eight, and the third in the least significant eight
bits. If there a fewer than three bytes to encode, the
corresponding buffer bits will be zero. The buffer is then
used, six bits at a time, most significant first, as indices
into the string
and the indicated character output. If there were only one or
two input bytes, the output is padded with two or one "="
characters respectively. This prevents extra bits being added
to the reconstructed data. The process then repeats on the
remaining input data.
Base 64 is used when transmitting binary data through
text-only media such as electronic mail, and has largely
replaced the older uuencode encoding.