The Radon-Hurwitz number $k(n)$ is the largest $k$ such that there exists an orthogonal multiplication $\mathbb R^k\times \mathbb R^n\to \mathbb R^n$; so for an ONB $x_1,\dots, x_k$
of $\mathbb R^k$ and a unit vector $y\in \mathbb R^n$ the vectors $y, x_1.y, x_2.y,\dots x_k.y$ are orthogonal in $\mathbb R^n$. This describes vector fields on $S^{n-1}$. The orthogonal multiplications were constructed by Radon 
[Lineare Scharen orthogonaler Matrizen, Abh. Math. Sem. Univ. Hamburg 1, 1-14, 1921] who extended the construction of Hurwitz for $k=n=1,2,4,8$. 
They extend to representations of Clifford algebras $C(\mathbb R^k, -\langle\quad,\quad\rangle)$ which explains the "periodicity" in $k(n)$ with respect to 8 and 2.
Adams showed that there are not more linearly independent vector fields on the sphere $S^{n-1}$.