Abbreviation
The label abbreviation refers to the practice of writing a word or phrase that could also be written out in full using only the letters of the alphabet. Examples are: Mr for mister, Dr for doctor and Capt for captain.
In British English full stops are uncommon after abbreviations that contain the first and last letters of the full expression. Examples are: Dr and Mr
In American English, full stops are common, and Mr. and Dr. are preferred.
An abbreviation usually does not have a distinct pronunciation of its own: we pronounce Mr as ‘mister’ and ‘Dr’ as ‘doctor’. There are, nevertheless, a few exceptions to this rule. For example, the abbreviation p. for pence is sometimes pronounced as ‘pee’.
Acronym
An acronym is a word constructed from the initial letters of the main words in a phrase. Examples are: LASER and SCUBA. An acronym can be pronounced as a word and it has the same meaning as the original phrase.
SCUBA for Self Contained Underwater Breathing Apparatus
LASER for Light Amplification by the Stimulated Emission of Radiation
NATO for North Atlantic Treaty Organization
Initialism
An intialism is a word constructed from the initial letters of the principal words in a phrase. It is formed in the same way as an acronym but cannot be pronounced as a word. Examples are: BBC for British Broadcasting Corporation and UNO for United Nations Organization.