@frostycab ...
an abbreviation : https://en.wikipedia.org/wiki/Abbreviation
[a shortened form of a written word or phrase used in place of the whole word or phrase.]
"Amt" is an abbreviation for "amount."
"USA" is an abbreviation of "United States of America."
ex : example / ie, e.g. ...
w/ = with
w/c = which [who cares]
w/n = with in
w/o = with out
etc / et cetera ... not Peter Cetera
idk [i don't know] ...
UK = unknown / [United Kingdom]
PI = Philippine Island
and so 4th ...
I suspect, did you skip your English/Vocabulary class