Synonymous, a.
Etymology: [Gr. Syn-, and Name.]
Having the character of a synonym; expressing the same thing; conveying the same, or approximately the same, idea. -- Syn*on"y*mous*ly, adv. These words consist of two propositions, which are not distinct in sense, but one and the same thing variously expressed; for wisdom and understanding are synonymous words here. Tillotson.
Syn. -- Identical; interchangeable. -- Synonymous, Identical. If no words are synonymous except those which are identical in use and meaning, so that the one can in all cases be substituted for the other, we have scarcely ten such words in our language. But the term more properly denotes that the words in question approach so near to each other, that, in many or most cases, they can be used interchangeably. 1. Words may thus coincide in certain connections, and so be interchanged, when they can not be interchanged in other connections; thus we may speak either strength of mind or of force of mind, but we say the force (not strength) of gravitation. 2. Two words may differ slightly, but this difference may be unimportant to the speaker's object, so that he may freely interchange them; thus it makes but little difference, in most cases, whether we speak of a man's having secured his object or having attained his object. For these and other causes we have numerous words which may, in many cases or connections, be used interchangeably, and these are properly called synonyms. Synonymous words "are words which, with great and essential resemblances of meaning, have, at the same time, small, subordinate, and partial differences, -- these differences being such as either originally and on the ground of their etymology inhered in them; or differences which they have by usage acquired in the eyes of all; or such as, though nearly latent now, they are capable of receiving at the hands of wise and discreet masters of the tongue. Synonyms are words of like significance in the main, but with a certain unlikeness as well." Trench.
Shades of meaning
Synonymous (Gr. syn, together, and onyma, name) strictly signifies being interchangeable names for the same thing, or being one of two or more interchangeable names for the same thing; to say that two words are synonymous is strictly to say they are alike, equivalent, identical, or the same in meaning; but the use of synonymous in this strict sense is somewhat rare, and rather with reference to statements than to words.
To say that we are morally developed is synonymous with saying that we have reaped what some one has suffered for us.
H. W. BEECHER Royal Truths p. 294. [T. & F. '66.]
In the strictest sense, synonymous words scarcely exist; rarely, if ever, are any two words in any language equivalent or identical in meaning; where a difference in meaning can not easily be shown, a difference in usage commonly exists, so that the words are not interchangeable. By synonymous words (or synonyms) we usually understand words that coincide or nearly coincide in some part of their meaning, and may hence within certain limits be used interchangeably, while outside of those limits they may differ very greatly in meaning and use. It is the office of a work on synonyms to point out these correspondences and differences, that language may have the flexibility that comes from freedom of selection within the common limits, with the perspicuity and precision that result from exact choice of the fittest words to express each shade of meaning outside of the common limits. To consider synonymous words identical is fatal to accuracy; to forget that they are similar, to some extent equivalent, and sometimes interchangeable, is destructive of freedom and variety.