Apostrophe: 
A figure of speech that directly addresses an absent or imaginary person, a personified  abstraction, or sometimes an inanimate object.

Example:

Milton! thou shouldst be living at this hour:
England hath need of thee: she is a fen
Of stagnant waters: altar, sword, and pen,
Fireside, the heroic wealth of hall and bower,

Have forfeited their ancient English dower
Of inward happiness. We are selfish men:
O raise us up, return to us again;
And give us manners, virtue, freedom, power.

Thy soul was like a Star, and dwelt apart:
Thou hadst a voice whose sound was like the sea,
Pure as the naked heavens, majestic, free;

So didst thou travel on life's common way
In cheerful godliness; and yet thy heart
The lowliest duties on herself did lay.

-William wordsworth

Explanation:

This poem has an apostrophe to Milton, one of the greatest writers of epic poems. William Wordsworth directly talks to Milton as if he knows him and they are best buds. This type of apostrophe adds an emotional intensity to the poem, which is what Wordsworth was going for. He also wants the reader to feel a sense of familiarity when reading the poem, this is why he refers to the apostrophe, Milton, as an old friend. The main purpose of the apostrophe is to address someone that is gone, but very important and needs to be remembered, which is exactly what Wordsworth does.