Acronyms and abbreviations

An acronym is a series of letters that stand for words. For example, PCR is an acronym that stands for polymerase chain reaction.

The most important thing is: always define an acronym before you use it. If you define it in the abstract of a scientific text, you need to define it again in the introduction.

Acronyms are a complex subject!

My advice: use acronyms made of capital letters without dots in between, like DNA, DVD, BBC, etc.

When you define an acronym from normal words, do not use capital letters at the start of the full words. 

For example:

The process of anaerobic digestion (AD) is used to make biogas.

When you define an acronym from a proper noun (name, etc), use capital letters. 

For example:

The World Health Organisation (WHO) recommends vaccinating children against polio.

Don't use small letters or dots in between.

Some exceptions:

One exception is certain biological words like mRNA. Another exception is acronyms that are not made of the just the first letter of a word (Interpol, International Criminal Police Organization).

Plurals:

When you use a plural, use a small "s" without an apostrophe.

For example:

Two CDs

Three DVDs

For unusual acronyms, specific styles, and other information, see the ridiculously long Wikipedia article on this topic.

History:

British English used to be very logical. If it was the 'British Broadcasting Company' it was abbreviated as 'B.B.C.' and if it was 'missing in action' it was 'm.i.a.'.  Big letter acronyms for big letter words, small letter acronyms for small letter words, and dots in between to make it clear it was an acronym.

Then the Americans decided that it would be easier to write these acronyms without all those annoying dots, making BBC and mia. But "mia" doesn't look like an acronym, it looks like a word, so they changed small-letter acronyms to big-letter acronyms to clearly show that they were acronyms. The British gradually agreed that this was more convenient and they started doing it too. That's why MIA is the acronym for 'missing in action', even though the original words have small letters.