A period (in America) is a full stop (.). It is a punctuation mark (.) at the end of a sentence. It shows that the sentence has finished. To be a sentence it must have at least one complete clause, with a verb and a subject.
When it is used in numbers, it is called a decimal point. It shows that the numbers to the right of it are less than one, as in 1.234. When it is used to write money in numbers, it shows that the numbers to the right of it are less than one pound/dollar/euro. For example:
- "Elizabeth'' bought a soda for $1.28 cents."
A point may be used after a word has been shortened: Pty. Ltd. The words doctor, mister, and mistress are often made shorter when used with a name. "Dr. Smith" is the name of a doctor whose last name is Smith, and "Mr." Banerjee is short for Mister Banerjee. "Mrs" means a wife, "Miss" is an unmarried female. "Ms" can be used for either. A point is also used after a first or middle initial, but does not follow the last name, for example, "J. Citizen".
When three periods are used together (...), they are called an ellipsis.
- Carey G.V. 1946. Mind the stop: a brief guide to punctuation with a note on proof-correction. Cambridge University Press, p13.