In most cases, the text content of a Tweet can contain up to 280 characters or Unicode glyphs. Some glyphs will count as more than one character. We refer to whether a glyph counts as one or more characters as its weight.
CEO, CoFounder at Higgs Boson Health
"Twitter developed in tandem with mobile internet, so it chose its character limits to be in keeping with those of traditional SMS. With a limit of 140 characters, it left 20 to spare to include the name of the sender, meaning tweets conformed to the limitations of text messages.