Electrical Meanings
An adapter or adaptor is a device that converts the physical form of a connector to another by converting characteristics of one electrical device or system to those of another that are otherwise incompatible. Some adapters change power or signal attributes; others simply adapt the physical form of a connector to another.
Other Meanings
This English language is full of mysterious words that confuse both native and non-native writers, and today’s two words are no exception. How exactly do you use adapter or adaptor? What is the difference between adapter and adaptor?
The term adaptor refers to a device for connecting devices that cannot be connected directly.
Example: I need a power adaptor to charge my laptop. When you turn on your laptop and are charging it, most power adapters feel warm to the touch. This is normal.
An Adapter refers to a person who adapts something.
Examples: A screenplay adapter feels the script needs some revisions. The plays were a very different tale: I worked with the adapter for 10 drafts.
Some people have claimed that there is a regional difference between these two words, and that adapter is used in American English and adaptor in British English (and some people cite the opposite).
Conclusion
In all contexts and in all forms of English around the world, both words can be used interchangeably. If you want to keep things tidy in your writing, use adapter in all cases. Fowler mentions that this is the most common spelling for both meanings in the English-speaking world, so following it is the best way to keep your audience’s attention on your message rather than your word choice.