Subtext is a literary term of art, and as concept, grew out of critical theories developed in the 1980s, specifically reader response criticism and early "close reading" theories.
It's a term that has increasingly moved to common parlance, but it is frequently not used in a universally agreed upon fashion. Here's the
American Heritage Dictionary entry for subtext:
AHD said:
1. An implicit meaning or theme of a literary text.
2. The underlying personality of a dramatic character as implied or indicated by a script or text and interpreted by an actor in performance.
Subtext is something that happens between the text and the reader. It should not be confused with the intention of the writer (see Wimsatt and Beardley's the Intentional Fallacy), but that does not mean that the writer did not create the subtext, nor does the existence of subtext mean that the writer is writing about him or herself.
Subtext is derived or arises from the text itself.
All texts are constructs, and they are completed or freed when they are read. The reader engages with the text to create meaning, but the meaning is derived from the author's words. Subtext lies between the lines, and is derive from metaphor and implication created by the writer.
As an example of subtext here's
John Donne's poem The Flea.
The poem sets up a scene wherein a man discusses a flea who bit first him, and then a woman:
Donne's The Flea said:
Mark but this flea, and mark in this,
How little that which thou deniest me is;
It sucked me first, and now sucks thee,
And in this flea our two bloods mingled be;
Thou know’st that this cannot be said
A sin, nor shame, nor loss of maidenhead,
Yet this enjoys before it woo,
And pampered swells with one blood made of two,
And this, alas, is more than we would do.
The flea sucked blood first from the man, and then the woman, and grew swollen from the blood.
The swollen flea is a metaphor or pregnancy, "swells with one blood made of two," a pregnancy which is not derived from sex or sin, or the loss of virginity. Yet becoming pregnant is "more" than the man and women wish to do (they don't want to conceive a child).
Yet Donne goes on:
Donne's The Flea said:
Oh stay, three lives in one flea spare,
Where we almost, nay more than married are.
This flea is you and I, and this
Our mariage bed, and marriage temple is;
Though parents grudge, and you, w'are met,
And cloistered in these living walls of jet.
Though use make you apt to kill me,
Let not to that, self-murder added be,
And sacrilege, three sins in killing three.
The pregnant flea becomes a metaphor for the man and the woman, joined in the flea by virtue of their blood mingling, so that they are "more than married." He elaborates the metaphor of them being joined in the flea to an image of them "cloistered in these living walls of jet."
"Use" means both custom and sexual intercourse; "use" was believed to shorten the lives of men. He then alludes to the holy trinity, saying that while use makes her "apt" to kill him, she should not kill the flea, and thereby kill him, herself and the flea.
In the final stanza Donne writes:
Donne's The Flea said:
Cruel and sudden, hast thou since
Purpled thy nail, in blood of innocence?
Wherein could this flea guilty be,
Except in that drop which it sucked from thee?
Yet thou triumph’st, and say'st that thou
Find’st not thy self, nor me the weaker now;
’Tis true; then learn how false, fears be:
Just so much honor, when thou yield’st to me,
Will waste, as this flea’s death took life from thee.
In the first two lines, Donne doesn't say she killed the flea, but he implies it quite clearly; her nail is purple with the blood of the flea. That's subtext.
The woman proclaims that the death of the flea has not harmed her or the speaker, and then, Donne agrees and counters with "Just so much honor, when thou yield’st to me, / Will waste, as this flea’s death took life from thee."
He never actually says "sleep with me. " Yet this entire poem is an attempt at seduction. The seduction is largely in the subtext, in the implications of the words, and it's largely born from metaphor.