literary device often used to reveal the innermost thoughts of a character. It is used to express a character’s thoughts about a certain character or past, present or upcoming event while talking to himself without acknowledging the presence of any other person. In a play, a character delivering a soliloquy talks to himself
— thinking out loud, as it were — so that the audience better understands what is happening to the character internally.

“O Romeo, Romeo!
Wherefore art thou Romeo ?
Deny thy father and refuse thy name;
Or, if thou wilt not, be but sworn my love,
And I’ll no longer be a Capulet.”
( Romeo and Juliet by William Shakespeare)
Juliet was thinking aloud about the traditional enmity between Romeo’s clan and her family, expressing her hopelessness about the success of their love.

is a literary device where a single character presents a speech in order to express his/her collection of thoughts and ideas aloud. Often this character addresses directly to audience or another character.

To be, or not to be, that is the question:
Whether 'tis nobler in the mind to suffer
The slings and arrows of outrageous fortune,
Or to take arms against a sea of troubles
And by opposing end them. To die—to sleep,
No more; and by a sleep to say we end
The heart-ache and the thousand natural shocks
That flesh is heir to: 'tis a consummation
Devoutly to be wish'd. -- Hamlet

a remark made in a low voice that you only intend particular people to hear
ROMEO
[Aside] Shall I hear more, or shall I speak at this?

an instruction in the text of a play, especially one indicating the movement, position, or tone of an actor, or the sound effects and lighting.

A tragic hero is a character in a work of fiction (o ften the protagonist) who commits an action or makes a mistake which eventually leads to his or her defeat.

A tragic flaw is a literary term that refers to a personality trait of a main character that leads to his or her downfall. In other words, a character with a tragic flaw is in need of some kind of attitude adjustment.
Romeo’s tragic flaw: thinking with his heart