an edge dislocation
Picture a thick book, all the pages stacked neatly. Now slip one extra half-page in from the top, so it wedges down partway and stops in the middle of the book. Right along the bottom edge of that extra half-page, the pages above are crowded together and the pages below have to bend and spread to make room. An edge dislocation is exactly this: a crystal that has one extra half-plane of atoms pushed in from one side, and the line where that half-plane ENDS inside the crystal is the dislocation. It is a line defect, a flaw that runs along a line rather than sitting at a single point.
Here is the precise picture. Take a perfect stack of atomic planes. Terminate one plane early, so it is present in the top half of the crystal but absent in the bottom half. The edge of that terminated plane is the dislocation line. Just above the line the atoms are squeezed (compression); just below, they are pulled apart (tension); and the atoms right on the line are badly out of place — this is the core, only a few atom-spacings wide. The defining measurement, the Burgers vector b, points along the direction the extra plane would have to close up, and for an edge dislocation b is PERPENDICULAR to the dislocation line. That perpendicular relationship is the signature of edge character.
Why does this matter? Because that extra half-plane can move. Push sideways on the crystal and the half-plane does not have to tear free all at once — it shifts its foot over by one atomic row, then the next, then the next, like a ruck travelling across a rug that lets you move a heavy carpet without dragging the whole thing at once. Each step needs only a tiny force. This is glide, and it is the reason real metals bend and yield at stresses ten to a hundred times smaller than a perfect crystal could withstand. The edge dislocation is where the softness of metals literally lives.
In a simple cubic model, remove the lower half of one vertical plane of atoms. The row of atoms sitting at the bottom edge of the remaining half-plane is the dislocation line, running into the page. Trace a small loop of atom-to-atom steps around that line and you finish one atom-step short of where you began; that closure gap, one lattice spacing long and horizontal, is the Burgers vector — at right angles to the line, as an edge dislocation demands.
An edge dislocation = one extra half-plane; its Burgers vector lies perpendicular to the line, and the line glides in the direction of b.
The extra half-plane is a common shorthand, but the dislocation is the LINE at its edge, not the whole plane. Whether the half-plane looks like it comes from the top or the bottom just flips the sign (positive vs negative edge) of the same defect.