As @Andrew said in the comment, we call it (all of them) abbreviation. However, if you want to be more specific,
There are four different types of abbreviations in English:
An initialism is formed from the first letters of a group of words.
We pronounce each letter individually.
FBI – Federal Bureau of Investigation
ASAP – as soon as possible
CD – compact disc
CEO – Chief Executive Officer
FAQ – frequently asked questions
PLC – public limited company
UFO – unidentified flying object
USA – United States of America
VAT – value added tax
An acronym is formed from the first letters of a group of words.
We pronounce the acronym as a word.
NASA – National Aeronautical and Space Administration
NATO – North American Treaty Organisation
OPEC – Organisation of Petroleum Exporting Countries
PIN – personal identification number (code for bank card)
RAM – random access memory (computer memory)
SIM – subscriber identification module (card for mobile phone)
VAT – value added tax (sales tax in the UK)
ZIP – zone improvement plan (post code in the USA)
VAT (value added tax) is a special case. It can be 2 types of abbreviation:
It can be an acronym and we pronounce it as one word /væt/ (rhymes with cat)
It can also be treated as an initialism and we pronounce each letter separately “v”, “a”, “t”
We must write acronyms with capital letters.
A shortening is an abbreviation in which the beginning or end of the word has been omitted.
There are 2 types:
Type 1 shortenings (treated as real words)
ad – advertisement (to promote a product or service)
app – application (software)
flu – influenza (an illness)
blog – weblog (a type of website)
rhino – rhinoceros (wild animal)
We use type 1 shortenings like real words.
We write them and say them as one word.
Rules for capital letters
The first letter is a capital letter only if the full word starts with a capital letter:
Example: full form is “Briton” (with a capital first letter) therefore the shortening also must start with a capital letter: “Brit”.
Example sentence: There are lots of Brits living in Spain.
Rules for full stops (periods)
We do NOT use a full stop after type 1 shortenings:
I placed an ad in the newspaper. correct
I placed an ad. in the newspaper. wrong
Type 2 shortenings (not treated as real words)
Feb. – February
Sat. – Saturday
Type 2 shortenings are only used in writing. But when we say or read them, we say the full version of the word.
Writing: “Please send me the Feb accounts.”
Speaking: “Please send me the February accounts.”
Rules for capital letters
The first letter of a type 2 shortening is a capital letter only if the full word starts with a capital letter.
Contractions are abbreviations in which we omit letters from the middle of a word. We do NOT write a full stop at the end of a contraction. The first letter is a capital letter only if the full word starts with a capital letter.
Type 1 contractions (missing letters from 1 word)
Dr – Doctor
govt – government
St – Saint
Mr – Mister
Type 2 contractions (missing letters from more than 1 word)
We use an apostrophe to represent the missing letters:
she's/he‘s – he is/she is
I'd/they‘d – I would/they would
I‘ve – I have
Source for abbreviations: