I’m rather surprised by the online OED’s definitions of assassinate and assassination.

It defines assassin as:

1. lit. A hashish-eater. Hist. (in pl.) Certain Muslim fanatics in the time of the Crusades, who were sent forth by their sheikh, the ‘Old Man of the Mountains,’ to murder the Christian leaders.

2. Hence: One who undertakes to put another to death by treacherous violence. The term retains so much of its original application as to be used chiefly of the murderer of a public personage, who is generally hired or devoted to the deed, and aims purely at the death of his victim.

It defines assassinate as:

1. trans & absol. To kill by treacherous violence.

2. trans. To endeavour to kill by treacherous violence; to attack by an assassin. Obs.

3. fig. To destroy or wound by treachery; to ‘stab’ reputation, etc.

It defines assassination as:

a. The action of assassinating; the taking the life of any one by treacherous violence, esp. by a hired emissary, or one who has taken upon him to execute the deed.

b. fig. Cf. ‘killing’.

I’m sure that most people asked to define assassination would say something along the lines of “murder for political or ideological motives”, and would agree that the motive is a defining characteristic - one does not ‘assassinate’ a business rival or an unfaithful husband. And although one can certainly hire (or be) a professional assassin, if one takes it upon oneself to kill one’s political or ideological opponent that is also assassination, surely. This isn’t a new sense: it’s how I have understood the word for my entire life (D.O.B. London 1956), and I find it very odd that motive isn’t mentioned in the definition at all, and only indirectly alluded to in description of assassin as “the murderer of a public personage”.

That definition was published in 1885 and probably written years earlier; I think we can safely assume that when the Third Edition gets around to the A’s in a few years, it will be more in line with current usage.

I agree entirely with Syntinen Laulu - assassination has always meant “murder for political [or religious - often the same thing] reasons”. That’s not just current usage. It might be interesting to take a close look at the OED’s cites for the word.

The OED’s description of the Assassins, which (according to SL) says “they were sent forth by their sheikh, the Old Man of the Mountains, to murder the Christian leaders”, is historically inaccurate.  The Assassins belonged to a Shi’ite Muslim sect ( the Isma’ilis), and their principal enemies, and by far most frequent targets, were Sunni Muslims. They were busy assassinating Sunni Muslim leaders long before the arrival of the Christians, and killed far more of these than they did Christians (with whom, in fact, they were quite willing to cooperate, when it suited their political ends).

I would venture to say that it may have always included that sense, but not been limited to it.  The definition-writers at the OED knew what they were doing.

See, for instance, the multiple uses of “assassin”, “assassinate”, etc., in Thackeray’s “The Case of Peytel”. It’s clearly being used in the “treacherous violence” sense, even with the motive of robbery.

Well done, Dr. Techie. Thackeray certainly uses the word “assassin” to mean “a treacherous murderer”, no more - no trace of political motive implied. You make a good case.

This is a prime example, I think, of the use of heavily loaded words words to advance the cause of a polemic, without bothering too much about their precise meaning, if any. One is reminded of Mr. Bryan, speaking of “crucifying mankind upon a cross of gold”. People do it all the time. 

To one whose “occupation as a writer” (save the mark!) has been confined largely to the careful description of events and facts - in technical papers, reports, SOP’s, etc., etc. - such a use of words may at times seem reckless, and can be unreasonably irritating.  I have found participation in wordorigins.org to be a salutary counterweight to the prescriptivist in me, who is constantly rearing his ugly head.