D-branes
D-branes are surfaces that "live" in string theory, and they come in various dimensions. D2-branes, for example, are two-dimensional and can also be called membranes. D0-branes are particle like and D1-branes are string like. Higher-dimensional objects can exist as well. D-branes are essential for making string theory mathematically consistent, and have far-reaching implications for a theory of quantum gravity.