|Definition - The dot character (.) that indicates the end of a sentence.|
1. Use them after imperative sentences that aren't exclamatory (i.e. that don't end with !).
Don't be late.
2. Use them after indirect questions.
Tell me how you did it.
3. If the sentence's final word ends with an abbreviation that uses a period, don't add another period.
I went to the market and bought some eggs, milk, etc.
4. Put periods inside parentheses if they contain a complete sentence.
Please read the afterword (It is enclosed as Appendix A.).
5. When in Europe, use them to indicate thousands.
1,317 (North America)
6. Don't use them with acronyms or initialisms.
7. Omit the ending period if it comes after a quotation mark that is preceded by a period.
He said: "She said: 'I know what it's like to be dead.'"
8. Omit them after Roman numerals that are being used as ordinals.
Super Bowl XVI
Etymology - The word period derives from the Latin periodus, a complete sentence, which in turn derives from the Greek periodos, rounded sentence or period of time (from peri, around + hodos, journey).
Note: Some claim that the punctuation mark was invented by Aristophanes of Byzantium.
Oxford English Dictionary - Its first citation in its punctuation-mark sense is from 1609:
"No Commaes but thy Stripes; no Periods But thy Nailes."
(J. Davies Holy Roode (1878) 20/2)