In this article, titled Holonomy Groups in Riemannian Geometry, on pg 37, the following line appears (before equation 2.5.3)
Let $\theta$ be a horizontal, orthogonal $1$-form on $\mathcal F_{GL}$
Here $\mathcal F_{GL}$ denotes the frame bundle of a smooth manifold $M$.
I know what a horizontal bundle of a fiber bundle means, and I know what a connection $1$-form on a principal bundle means. But I am unable to see what is meant by a "horizontal orthogonal $1$-form". Can somebody please explain what this means?