Character
A unit of text. Complicated in modern contexts — "character" can mean byte, codepoint, grapheme, or glyph depending on context.
ASCII (1963): 7 bits, 128 characters, only enough for English. UTF-8 (1992): 1–4 bytes per codepoint, backward-compatible with ASCII, default on Linux/web. A Unicode codepoint ≠ character — "👨👩👧👦" is 4 emojis joined with ZWJ (Zero-Width Joiner) but one grapheme. So string "length" is ambiguous. Swift's "Character" = grapheme; JavaScript's "string.length" = UTF-16 code units (can give odd answers for emoji).