In A Wrinkle in Time, what is a tesseract?

To tesseract is to travel through the fifth dimension, through time and space. It is a wrinkle in time that allows travel to distant planets in a short time. When tessering, Meg feels like she is in a void without sight, sound or feeling. Coming out of a tesser brings a tingling feeling into the body. When Meg tessers through the Black Thing, in addition to the dark, she feels a sense of coldness and despair that tries to grab her. Mrs. Which is an expert at tessering. Meg's father can tesser, but he cannot control where he goes.