Equanimity — the Stoic ideal of tranquillity — is one of the most misunderstood concepts in philosophy. It is commonly confused with emotional flatness, with indifference, or with the kind of stoic (lowercase 's') suppression of feeling that the word "stoic" has come to mean in popular usage. None of these is what the Stoics meant by equanimity.
The Stoic concept of equanimity — ataraxia in Greek, tranquillitas animi in Latin — is a specific quality of mind: stable, clear, and genuinely responsive to the world without being overwhelmed by it. It is not the absence of feeling but the governance of feeling — the capacity to experience the full range of human emotion without being destabilised by it.