design point

A design point \mathbf x=(x_1,x_2,\dots,x_n)\in\mathbb R^n collects a complete choice of every decision variable into one vector — ordinary vector notation throughout, with the name emphasizing that it represents one specific candidate solution rather than a direction or displacement. Several design points are frequently stacked as the rows of a matrix X (the design matrix, one row per design point and one column per variable), the same convention Linear Algebra uses for a regression model's data matrix.