Definition of "hi"

hi¹

/ˈhaɪ̯/

interjection
1. A friendly, informal, casual greeting said upon someone's arrival.
  Quotations
  Anna: Pete, hi! Hi, we are here! — Pete: Hi, Anna! Hi, Marsha! — Anna: Hi! — Pete: How are you two? — Marsha: I am great! Audio (US)(file)
  2016, VOA Learning English (public domain)
2. An exclamation to call attention.
  Quotations
  I made a speaking trumpet of my hands and commenced to whoop “Ahoy!” and “Hello!” at the top of my lungs. […] The Colonel woke up, and, after asking what in brimstone was the matter, opened his mouth and roared “Hi!” and “Hello!” like the bull of Bashan.
  1913, Joseph C. Lincoln, chapter 7, in Mr. Pratt's Patients
  'Come back now!' shouted Sam. 'Hi! Come back!' But Gollum had vanished.
  1954, J. R. R. Tolkien, The Two Towers
3. (dated) Expressing wonder or derision.
noun
plural his
1. The word "hi" used as a greeting.

hi²

adjective
1. Informal spelling of high, often in hyphenated terms.