Reading Negative Dialectics, I was unable to track down a succinct, suitable definition of the terms "intentio recta" and "intentio obliqua", first appearing on page 69 of the Ashton translation.
It turns out the phrases are derived from the work of Nicolai Hartmann who introduced the terms to correspond to the Scholastics' intentio prima and intentio secunda. The intentio recta, therefore, is the state when cognition focuses upon the true object, while intentio obliqua is a state of consciousness which focuses upon the image of the object in the intellect.
In the neo-Kantian schema, this refers to whether we know the thing-in-itself ("intentio recta") or the image of the thing ("intentio obliqua").
Featured image by Пероша under a CC-BY-NC license.