The line "O Captain! My Captain" off Walt Whitman's poem, addressed to the assassinated President Lincoln is indeed an example of an apostrophe. A apostrophe is a literary device used when a speaker breaks off from addressing someone and addresses a third party that can be present or absent. It is mostly used in drama, but it also appears in poetry. In this example, the speaker of the poem begins with an address to Lincoln, the dead captain.
I checked the definitions of these words in a dictionary. You can also find the definitions of these words in a dictionary or on the internet if you require further explanation.
