Given that $y=kx+d$ is called a linear relationship, are there established naming conventions when more variables are concerned?
The linear equation can be generalized by $b + \sum a_ix_i = 0$ and the result should be a hyperplane. In the 2D case mentioned at the beginning, such a hyperplane is a line and I assume that linear relationship is derived from that. In the 3D case $z = ax + by +c$, the hyperplane results in a simple plane (example drawing), is that relationship called planar, analogous to the linear example?