A labeled dataset is linearly separable when one straight boundary can place its classes on opposite sides. In one dimension, that boundary is a point; in two dimensions, it is a line. When no such boundary exists, adding features can sometimes make the transformed dataset separable by a plane while producing a nonlinear boundary after projection. section 11.3
From a line to a curve
A dataset arranged in a circular pattern cannot be separated by a line. Raising each point by its squared distance from the origin creates a three-dimensional dataset that a plane can separate; projecting that plane back produces the circular classifier x1² + x2² = 1. Similarly, adding x1x2 helps separate a modified XOR dataset.


