Romanization of Greek is the transliteration (letter-mapping) and/or transcription (sound-mapping) of text from the Greek alphabet into the Latin alphabet.
The conventions for writing and romanizing Ancient Greek and Modern Greek differ markedly. The sound of the English letter B (/b/) was written as β in ancient Greek but is now written as the digraph μπ, while the modern β sounds like the English letter V (/v/) instead. The Greek name Ἰωάννης became Johannes in Latin and then John in English, but in modern Greek has become Γιάννης; this might be written as Yannis, Jani, Ioannis, Yiannis, or Giannis, but not Giannes or Giannēs as it would be for ancient Greek. The word Άγιος might variously appear as Hagiοs, Agios, Aghios, or Ayios, or simply be translated as "Holy" or "Saint" in English forms of Greek placenames.
Traditional English renderings of Greek names originated from Roman systems established in antiquity. The Roman alphabet itself was a form of the Cumaean alphabet derived from the Euboean script that valued Χ as /ks/ and Η as /h/ and used variant forms of Λ and Σ that became L and S. When this script was used to write the classical Greek alphabet, ⟨κ⟩ was replaced with ⟨c⟩, ⟨αι⟩ and ⟨οι⟩ became ⟨æ⟩ and ⟨œ⟩, and ⟨ει⟩ and ⟨ου⟩ were simplified to ⟨i⟩ (more rarely—corresponding to an earlier pronunciation—⟨e⟩) and ⟨u⟩. Aspirated consonants like ⟨θ⟩, ⟨φ⟩, initial-⟨ρ⟩, and ⟨χ⟩ simply wrote out the sound: ⟨th⟩, ⟨ph⟩, ⟨rh⟩, and ⟨ch⟩. Because English orthography has changed so much from the original Greek, modern scholarly transliteration now usually renders ⟨κ⟩ as ⟨k⟩ and the diphthongs ⟨αι, οι, ει, ου⟩ as ⟨ai, oi, ei, ou⟩.
"Greeklish" has also spread within Greece itself, owing to the rapid spread of digital telephony from cultures using the Latin alphabet. Since Greek typefaces and fonts are not always supported or robust, Greek email and chatting has adopted a variety of formats for rendering Greek and Greek shorthand using Latin letters. Examples include "8elo" and "thelw" for θέλω, "3ava" for ξανά, and "yuxi" for ψυχή.