From instant + -ly.

• (British) IPA: /ˈɪnstəntli/

instantly (not comparable)

1. At once; without delay. [from 16th c.]

   When the neighbours' dog barked, ours instantly replied with a howl.

   • 1897 December (indicated as 1898), Winston Churchill, chapter V, in The Celebrity: An Episode, New York, N.Y.: The Macmillan Company; London: Macmillan & Co., Ltd., →OCLC ↗:

     Then we relapsed into a discomfited silence, and wished we were anywhere else. But Miss Thorn relieved the situation by laughing aloud, and with such a hearty enjoyment that instead of getting angry and more mortified we began to laugh ourselves, and instantly felt better.

2. (archaic) Urgently; with insistence. [from 15th c.]
   • c. 1602, William Shakespeare, “The Tragedie of Troylus and Cressida”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act I, scene ii]:

     Sir, my lord would instantly speak with you.

   • 1603, Michel de Montaigne, chapter 27, in John Florio, transl., The Essayes […], book II, London: […] Val[entine] Simmes for Edward Blount […], →OCLC ↗:

     Theoxena although she were instantly urged thereunto, could never be induced to marry againe.

3. (obsolete) At the same time.
   • c. 1597 (date written), William Shakespeare, “The First Part of Henry the Fourth, […]”, in Mr. William Shakespeares Comedies, Histories, & Tragedies […] (First Folio), London: […] Isaac Iaggard, and Ed[ward] Blount, published 1623, →OCLC ↗, [Act V, scene ii]:

     As if he mastred there a double spirit
     Of teaching, and of learning instantly

• (at once; without delay) immediately, instantaneously

• French: instantanément
• German: unmittelbar
• Portuguese: instantaneamente
• Russian: сра́зу
• Spanish: al instante, al hilo, al tiro, instantáneamente

• French: instamment

1. As soon as, directly.

   I phoned instantly I heard you were back.