Definition:Direction Cosine

Definition
Let $\mathbf a$ be a vector quantity embedded in a Cartesian $3$-space.

Let the angles which $\mathbf a$ makes with the $x$-axis, $y$-axis and $z$-axis be $\alpha$, $\beta$ and $\gamma$ respectively.

Then the direction cosines of $\mathbf a$ are $\cos \alpha$, $\cos \beta$ and $\cos \gamma$, defined individually such that:


 * $\cos \alpha$ is the direction cosine of $\mathbf a$ with respect to the $x$-axis


 * $\cos \beta$ is the direction cosine of $\mathbf a$ with respect to the $y$-axis


 * $\cos \gamma$ is the direction cosine of $\mathbf a$ with respect to the $z$-axis.

Also presented as
Some sources do not dwell on the actual angles themselves, but instead denote the direction cosines directly as $\alpha$, $\beta$ and $\gamma$.

While this technique results in more streamlined notation, it can result in confusion.