In its narrowest sense, a synonym is a word or phrase that is perfectly substitutable in a context for another word or phrase. People who study language professionally agree that there is no such thing as an ideal synonym, for it is virtually impossible to find two words or phrases that are identical in denotation (meaning), connotation, frequency, familiarity, and appropriateness. Indeed, linguists have long noted the economy of language, which suggests that no language permits a perfect fit, in all respects, between any two words or phrases. Many examples of overlapping can be cited; the more obvious ones in English are those that reflect a duplication arising from Germanic and Romance sources, like motherly and maternal, farming and agriculture, teach and instruct. In such pairs the native English form is often the one with an earthier, warmer connotation. In some instances, where a new coinage or a loanword has been adopted inadvertently duplicating an existing term, creating 'true' synonyms, the two will quickly diverge, not necessarily in meaning but in usage, application, connotation, level, or all of these. For example, scientists some years ago expressed dissatisfaction with the term tidal wave, for the phenomenon was not caused by tides but, usually, by submarine seismic activity. The word tsunami was borrowed from Japanese in an attempt to describe the phenomenon more accurately, but it was later pointed out the tsunami means 'tidal wave' in Japanese. Today, the terms exist side by side in English, the older expression still in common use, the newer more frequent in the scientific and technical literature. Any synonym book must be seen as a compromise that relies on the sensitivity of its users to the idiomatic nuances of the language. In its best applications, it serves to remind users of words, similar in meaning, that might not spring readily to mind, and to offer lists of words and phrases that are alternatives to and compromises for those that might otherwise be overused and therefore redundant, repetitious, and boring. The Oxford Thesaurus goes a step further by offering example sentences to illustrate the uses of the headwords and their alternatives in natural, idiomatic contexts