The nouns parricide and patricide share the definitions (1) the murder of one’s father, and (2) a person who murders his or her father. But parricide is sometimes used more generally, referring to the murder of one’s mother, grandparent, or other close relative.
Both words came to English around the 16th century, and both have Latin roots. Pater is the Latin word for father, and -cide is a Latin suffix meaning slayer or murderer. Parricide, which comes to English via French from the Latin parricīda, shares the -cide suffix,1 but the origins of parri are more mysterious. Different authorities say different things, but there’s a good chance parri relates to peos, a Greek word for kinsman.2
But for the most part, the words are variants of each other. Most dictionaries list parricide as the primary form, and the word appears about twice as often as patricide. And though patricide may be the more etymologically justifiable term for the murder of one’s father, parricide is often the term for that crime in legal contexts. Patricide is more often used in the sense one who murders his or her father (at least in historical texts up to the 20th century—instances of the word used this way in this century are difficult or impossible to find).
Related words: Matricide is the killing of one’s mother. Fratricide is the killing of one’s brother. And the words sororicide (killing one’s sister) and filicide (killing one’s offspring) are found almost exclusively in dictionaries and other references sources.
The ngram below graphs the use of parricide and patricide (as a percentage of all words) in a large number books, magazines, and journals published between 1800 and 2000. Of course, the graph might not tell us much because it doesn’t show how the words are used, but it’s nevertheless interesting that patricide has recently gained ground while parricide has become significantly less common.
It is observable that he made no law against parricide, as supposing it a crime that could never exist in any community. [The Grecian History, Oliver Goldsmith (1805)]
They even whisper that because he has slain Monseigneur, and because Monseigneur was the father of his tenants—serfs—what you will—he will be executed as a parricide. [A Tale of Two Cities, Charles Dickens (1868)]
For some crimes, such as patricide — which is the worst crime known in Chinese criminology — a whole city might be destroyed and thousands made to suffer and bear a disgrace that would last for generations. [The World's Work, Walter Hines Page and Arthur Wilson Page, (1910)]
The mosl powerful of these conflicts weu the infantile urge to parricide and incest — the Oedipus ant Electra complexes. [New York Times (1961)]
Reports disclosed that the complainant … filed an attempted parricide case against his son. [Zambonaga Today Online (2011)]
Ms. Merkel called for Mr. Kohl to step aside, in a front-page letter to Germany’s main conservative paper. The audacity of this patricide, never forgiven by Mr. Kohl, secured her leadership of the CDU. [Wall Street Journal (2012)]