ES Ratio stands for Eye Separation Ratio. It is the interpupillary distance (IPD, the distance between the pupils) divided by the bizygomatic width or the distance between the widest protrusion of the cheekbones.
This ratio has been proposed to be an important component of facial attractiveness, with the ideal ES ratio being around 0.46, according to Pollet et al.. They measured the influence of manipulating this ratio on a series of computer-generated female faces on attractiveness judgements. They concluded that this ratio might be the reason why 'averaged' (a series of faces combined into a composite image) facial portraits tend to be perceived as more attractive than standard faces because averaged faces tend to converge towards an ideal ES ratio.