What is the definition of $[\alpha/{\rm H}]$? The notation $[A/B]$ denotes
\begin{equation}
[A/B] = \text{log}_{10}\left[\frac{(\text{number of A atoms/number of B atoms})_{\star}}{(\text{number of A atoms/number of B atoms})_{\odot}}\right]
\end{equation}
or
\begin{equation}
[A/B]\equiv\text{log}_{10}\left(\frac{A}{B}\right)_{\star}-\text{log}_{10}\left(\frac{A}{B}\right)_{\odot}
\end{equation}
where $\star$ refers to the star and $\odot$ refers to the Sun. $[A/B]$ means the log abundance of $A/B$ relative to solar. 
Therefore, $[{\rm Fe}/{\rm H}]$ is the log abundance of ${\rm Fe}/{\rm H}$ relative to solar. In a star with $[{\rm Fe}/{\rm H}]=-2$, iron is 1% as abundant as in the Sun. 
In SDSS SEGUE papers, one finds $[\alpha/{\rm Fe}]$. What does this $\alpha$ denote? Helium?
 A: The $\alpha$ refers to "alpha process elements" which are the result of a class of fusion processes involving helium. The $\alpha$ elements are roughly speaking a bunch of helium nuclei ("$\alpha$ particles") stuck together, so they have an integer multiple of 4 nucleons. The $\alpha$ elements are Ne, Mg, Si, S, Ar, Ca and Ti.
A measurement of any one of these elements can be called "$\alpha$", it's not some combination of their abundances. Lumping them together means that measurements that use different tracers can be combined in one comparison. For instance, Type II supernovae produce a certain ratio of $\alpha$ elements and "iron peak" elements (Ti, V, Cr, Mn, Fe, Co and Ni), while Type Ia supernovae produce primarily iron peak elements. The metallicity (e.g. [Fe/H]) at which [$\alpha$/Fe] begins to drop is thus indicative of when in a galaxy's history Type Ia supernovae began contributing to chemical enrichment.
A: Nevermind, I found the answer here:
https://arxiv.org/abs/1010.2934
"The chemical elements O, Mg, Si, Ca, and Ti, which are often referred to as the “$α$” elements"
