There is no exact synonym for "the".
In more ordinary contexts, an appropriate demonstrative pronoun -- one of "this", "that", "these", or "those" could usually be used instead of "the" and have almost the same meaning.
But in certain contexts, primarily legal ones, "said" can be used as a synonym for the.
For example, a patent claim might contain the phrase, "... a square rod, an electrical brush in contact with said square rod, ..." instead of "... a square rod, an electrical brush in contact with the square rod, ..." .
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A synonym for "shell" can be "hull," which refers to the outer covering or structure of something, particularly in relation to boats or containers. Other synonyms include "case," "cover," or "enclosure," depending on the context in which the word is used.