A text-based data encoding that allows binary data to be sent over channels which only support ASCII text (e.g. teletype systems), at the cost of a 33% increase in size. Standardized by RFC 4648. Often recognizable by the presence of one or two equals signs ([code]=[/code]) at the end of the text, though this is not possible when padding characters are ommitted, or when the length (in bytes) of the original data is a multiple of 3.
YouTube notably uses a form of this encoding - specifically "Base 64 Encoding with URL and Filename Safe Alphabet" (RFC 4648 §5) - for its video IDs, with padding characters ommitted. For example, [code]dQw4w9WgXcQ[/code] is the video ID for Never Gonna Give You Up.
More recently popularized by Doki Doki Literature Club, which used Base64 in several of its easter eggs.