Which sandhi is present in 'मतैक्य'?

The word 'मतैक्य' is formed by the combination of:
$$\text{मत}+\text{ऐक्य}=\text{मतैक्य}$$

According to the rules of Vriddhi Sandhi (वृद्धि संधि), when 'अ' or 'आ' is followed by 'ए' or 'ऐ', they combine to form 'ऐ'. Here, $\text{अ}+\text{ऐ}=\text{ऐ}$.