I am aware of the dictionary definition of the word "immortal": (source)
Or in words:
living forever; never dying or decaying
However, that does not directly imply "can't be destroyed", as far as I can see. My logic (which might be wrong!) tells me that someone might live forever by not aging or ever being sick, i.e. never die of natural cause but still, can be killed by unnatural causes.
This logic is supported by numerous mythology related films I've watched over the years, where gods of all kinds (and a God should be immortal, right?) are killed and destroyed, e.g. it's even mentioned in this question on a sister site on SE.
So my question is: can the word "immortal" in plain speech or writing, be used to describe something that still can be killed or destroyed? If not, how do you explain this inconsistency, language-wise?
Best Answer
Yes, and that's in fact what "immortal" mostly has meant.
The etymology of the word "immortal" reveals
The meaning of the word "dead" itself is
When we try to compare the meaning according to the definitions of related words, we deduce that you can't. However, irregardless of antonyms and synonyms, with regards to usage, a word might have a slight meaning difference to what is implied.
The controversy lies in the fact that being killed also results in someone's death, but someone may die due to senescence, and "not by an external force".
Being killed has always been undesirable, but it has not mainly been what humans dreamed of avoiding. Escape from the inevitable death caused by old age, however, has been the subject of much of the fiction in human history. That there are many immortal Greek figures that are kill-able, but do not die of senescence, proves this.
When you want to talk about something fictitious, you decide what it means. So you can define immortality the way you want. So you may define your legendary creature as one that doesn't even die due to injuries etc. Or, you could take a look at nature.
The closest nature has to immortality is the doesn't-die-by-senescence version: Biological immortality has gained recent interest among researchers. See also a question about Immortal organisms on biology.SE.
Bottom line is, you can. The implied meaning has usually been very close to what you think. Vampires have sometimes been absolute im-mortals, sometimes beheading killed them, and sometimes silver bullets.