eth

noun1

1. •
   A letter (capital Ð, small ð) introduced into Old English to represent its dental fricative, then not distinguished from the letter thorn, no longer used in English but still in modern use in Icelandic, the IPA and other phonetic alphabets to represent the voiced dental fricative "th" sound as in the English word then. The letter is also used in Faroese, but is generally silent in that language.

   • In Old English manuscripts thorn and eth did not have different phonetic values but were used positionally[.] — 1985

noun1

1. •
   (UK) Initialism of electric train heat/heating.