A homoglyph is a character that looks identical (or nearly identical) to another but is technically a different character with a different code point. Classic examples are the Cyrillic small letter "а" (U+0430) versus the Latin "a" (U+0061), the digit 0 versus the letter O, or the pair rn reading as m.
Attackers use homoglyphs to register domains that are visually indistinguishable from a real brand while resolving to attacker-controlled infrastructure — a homoglyph or IDN homograph attack.
Homoglyph lookalikes are detected with the Unicode Consortium's confusables data (UTS #39), which maps each character to a canonical skeleton; two strings with the same skeleton are confusable. See the guide on homoglyph and IDN lookalike domains.