D-brane

D-branes are entities in string theory. D-brane is short for "Dirichlet membrane." These are extended objects upon which open string end points lay on that obey Dirichlet boundary conditions. 

It was shown in the 1990s by Joseph Polchinski and others that string theory requires the presence of these higher dimensional entities. Polchinski called these higher dimensional objects "D-branes" and identified them with the black hole black p-brane solutions of supergravity. These objects opened up a rich new field of rich mathematical structure. 

D-branes are classified by their spatial dimension. This value is indicated by a number after the D. 

D0-brane is a single point.

D1-brane is a line segment or a D-string.

D2-brane is a plane.

D25-brane is the highest possible spatial dimension in bosonic string theory. The 26th dimension is temporal.