Ea is the term for activation energy, what is the mean of a in this term?
The subscript $\mathrm a$ in $E_\mathrm a$ may refer to Arrhenius.
The IUPAC gold book states that:


  activation energy (Arrhenius activation energy)
  An empirical parameter characterizing the exponential temperature dependence of the rate coefficient, $k$, $E_\mathrm a = RT^2 \frac{\mathrm d(\ln k)}{\mathrm dt}$,
  where $R$ is the gas constant and $T$ the thermodynamic temperature.