Early regressions used what we would now call "non-linear methods" that were iterative and quite tedious to manually perform before the invention of computers, electronic calculators or even publication of books with logarithm tables. Using derivative calculus, it was found that fitting equations which were linear in their coefficients, such as polynomials, could be directly solved using linear algebra. So the name "linear regression" historically refers both to the linear algebra and the type of equations that can be used with it - and a great cheer rose up throughout the world as well. Linear equations can also be solved by iterative, non-linear methods, but there is seldom reason to do so now.