It is called both spine and spline, although spline is the more accurate term because it is describing the stiffest or least bendable side or plane of the blank. In other words, the spline is the side of the blank most resistant to bending (some folks think this also means it is the strongest side, but that is eroneous). It is found by either 1) placing the butt of a rod section (the thicker portion of the section) on a table, allowing the top or thinner portion of the section to rest on the 1st two fingers of one of your hands, and then pushing or flexing the blank down in the center of the section while slowly rolling it as you push down on it; 2) or by placing the butt (thicker portion of the rod section) againse the bottom edge of a table and pushing up in the center of the blank against the table and your other hand. The spline is located where the blank "jumps" or "resists bending".
However, as valuable splining is with bamboo or greenheart rods, its value with fiberglass, boron, or graphite is a matter of much debate. Supposedly rods with guides placed on or opposite the spline will have the rod blank track in a straighter plane when casting than those with the guides placed elsewhere. With wood rods (bamboo or greenheart) this makes perfect sense because of their much lower modulus and elasticity; but with modern tubuler fiberglass, boron, and graphite it is very difficult to quantify the difference (especially with the high modulus graphites) because they are so stiff and recover so fast the difference in tracking are rather minute.
That is why splining or not splining and putting guides on, opposite, or 90 degrees to the spline are each advocated by rod builders. It is also the reason some say to simply place the guides on the rod's straightest plane as you sight down the blank section and not worry about the spline (this is what Sage does).