A "semantic shift" is the same as a "semantic change." In terms of linguistics, it appears that it is simply a "shift" in the meaning of a word relative to the time it is used. Some words in Hamlet, by William Shakespeare, for example, may have been used one way in the past, but are used very differently today, or not at all. E.g., in a more modern context, for example, "bad" used to mean "awful," but with the advent of slang, and Michael Jackson's song of the same name, "bad" came to mean "cool" or "tough." This is one of many reasons that the English language is so difficult to understand and/or learn. What we say is not always what we mean.
In terms of writing of the past, what was said then is not always perceived today in the context of how it was used in days gone by. It is often easier to understand a word when we can look at its etymological development: the history, meaning and "shifts" a word has gone through.
As an example, looking at etymologies, we can understand from what language a word evolved, or how it was created in the first place. Look at the word "escape." It comes from the old French, around 1300. This is its etymology or semantic history:
c.1300, from O.N.Fr. escaper, O.Fr. eschaper (12c., Mod.Fr. échapper), from V.L. excappare, lit [literally], "get out of one's cape, leave a pursuer with just one's cape, ...."
The idea was that if two men were fighting, one might grab the other by his cape, whereby the man captured needed only to release the clasp of his cape and he was free, while his attacker held only a cape in his hands. Etymologies can be fascinating. The growth of and change in words is also seen in "semantic shifts." In Hans Heinrich Hock's 1986 publication, Principles of historical linguistics (Berlin: Mouton de Gruyter), he states:
historical linguistics cannot ignore semantic change. For unless we can relate words such as Old English hlāf ‘bread’ and New English loaf not only phonetically but also semantically, it is impossible to trace many historical developments....
In looking at Hamlet, we are faced also with "the historical development of words." In the context of Shakespearean English, we often find it difficult to understand unless we alter the order of the words, or look into meanings of words that are considered archaic today.
As an example, in Act Three, scene one, in Hamlet's famous "To be or not to be" speech, we find a word that is used differently then than it is today. Hamlet says,
...When we have shuffled off this mortal coil... (III.i.74)
This literally means, "after we have cast away the 'hurly burly' or confusion of our mortal lives." Coil is used in a much different manner in modern speech. In Hamlet, "mortal coil" means "life," but when we speak of coils today, there is no such connection. Today, coil used as a noun means something tightly wound, like a Slinky:
"a connected series of spirals or rings into which a rope or the like is wound..."
Whereas an Elizabethan audience would understand the language, so many years later, it is lost to a modern audience. This is also why comic relief used in Shakespeare's plays is often not as amusing to a modern audience, or why allusions made during that era are not timely or effective today. It is because of the separation of time, and the semantic shift, that the way in which words are used today is different than in the past.
As another example, in the same speech, Hamlet refers to "despised love" which we would believe to mean "hated" love, however in translation today, it means "unrequited" or unreturned love. (79)