Unflappable is an adjective used to describe someone who remains calm and composed in the face of difficulty or stress. Synonyms for unflappable include cool-headed, level-headed, collected, unemotional, unperturbed, poised, serene, composed, stoic, and placid. These words describe someone who maintains their composure and doesn't let their emotions get the best of them, even in the most challenging situations. Being unflappable is a highly-regarded trait and is often associated with leadership, professionalism, and mental strength. Using these synonyms in place of unflappable can add variety and depth to one's writing, while still capturing the essence of someone who remains unruffled in the face of adversity.