When the IAU (International Astronomical Union) says so, apparently. That body of eminent astronomers decided, on 2006-Aug24, that a dwarf planet is not a planet. And that sort of buggering up of the English language concerns me.
The way adjectives work in English is simple - a child can understand it, but highly educated astronomers seemingly need to have it explained. Something that is a red shoe is also a shoe. A blue shoe is a shoe. A big shoe is a shoe. A small shoe is a shoe. A large pizza is a pizza. A small pizza is a pizza. And English works that way, not just for shoes and pizzas, but for all nouns.
Ergo, a dwarf planet is a planet. Saying otherwise is absurd, is nonsense, and will lead to much more that is nonsense.
It is not that I want these tiny bodies to be planets. Indeed I do not! But I do feel forced to point out that a new name is required for this category of objects in order for them not to be planets. Several candidate terms have been suggested, including pluton, planetoid, planetino, and any of them would work for English. (I understand "pluton" fails miserably in French and a few other languages where it is how they spell what we call Pluto.) I am torn as to whether a word made from "planet" would be superior to or inferior to one that isn't. In any event, either planetoid or planetino would be better than dwarf-planet (with a hyphen); whereas dwarf planet (as two words) is simply not a candidate.
For many decades, some people have been saying anti-semitic when they mean anti-jewish. (I don't know exactly what went wrong here, at least not for every such person, but possibly this was begun by mealy-mouthed individuals who thought "Jew" a derogatory term?) The absurdity of what follows from such misuse of language was brought home to me when I heard an Arab, who clearly loved his own people, being called an anti-Semite.
Although this may seem at first glance a minor point, it involves a horrible insult to all non-jewish Semites. If we acknowledge that one of the meanings of anti-semitic is anti-jewish, as many otherwise reputable dictionaries do, we are saying Semite means Jew, thereby defining into non-existence all those who are semitic but non-jewish. No wonder the Arabs feel the rest of the world hates them! Having US-American bombs dropped upon them may also contribute to this feeling.
And that brings me to another such misuse of language, one that is extremely wide-spread, is insulting to Canadians, to Mexicans, as well as to the inhabitants of all Central- and South-American countries. It is the use of "America" to mean USA, and of "American" to mean US-American. Clearly "America" is and has to be a collective term for the two continents North America and South America. I see no excuse for using "America" to refer to one particular country within those continents, since that country has many perfectly acceptable names, United States, United States of America, USA, US, to name a handful. However when we need to construct the adjective to denote being of or from that country then we encounter a difficulty; I'm unable to come up with anything better than US-American or USAmerican or U-S-American, yet I doubt it will catch on, in any of those hyphenations. Hmm, I thought this my own linguistic innovation, albeit many years ago, but googling informs me that it currently appears in the great Wikipedia and in other places, in both singly- and un-hyphenated forms as well as in an ill-advised two-word form. All these uses on the world-wide-web appear to be in recently written pieces, so such usage, no matter who coined it (and it may well have been coined by multiple people independently), is clearly on the rise, and that is a trend I find encouraging.
One also finds Usian, Usanian, Usonano (in Esperanto), United-Statesian, United-Statian (reportedly equivalent to estadounidense in Spanish?), Yank (or Yankee, or yanqui in Spanish) being used or at least suggested; and the Germans got it just right with their US-Amerikaner -- aha, so that is where it comes from. Spanish-speakers seem perfectly clear that americano means anyone from the Americas in the broadest sense. For much more on the subject you would do well to start from Wikipedia: Use of the word American. If you seek only a little more on the subject, I can recommend a light-hearted piece: The trouble with Americans - from the Guardian.
[The preceding implies Mexico is part of North America, while acknowledging the existence of a geographic entity known as Central America, and yet denying it continent-status. But I'll leave all that for another day.]
I confess that I found it merely amusing when in the 1960's IBM named their operating-system Operating-System, and later when they named their design for a personal computer Personal Computer. Mind you, those bits of linguistic chauvinism were perfectly harmless, since we were going to call that operating-system OS, and those small computers PC's:-) We also pronounced CICS as "kicks" despite IBM insisting it was to be pronounced C-I-C-S. And I, for one, pronounce Apple's newest operating system as O-S-X despite Apple insisting it be said O-S-ten. They say that's obviously the only correct pronunciation because in OS-X, which came after OS-9, the "X" is the Roman-number ten. I'll grant them the last part, but I ask you: does one pronounce the Roman-number XII as "ten-I-I"?