【Equanimity】 , 【composure】 , 【sangfroid】 , 【phlegm】  mean the mental temper of one who is self-possessed or not easily disturbed or perturbed.

【Equanimity】 suggests either a proper mental balance or a constitutionally equable temper; it therefore may imply either a delicate adjustment of one’s emotional and mental powers that is liable to disturbance only under great strain or a settled attitude of mind which repels all that disturbs.

【Composure】 commonly implies the conquest of mental agitation or disturbance by an effort of will, though it may imply a temperamental freedom from agitation.

【Sangfroid】 implies great coolness and steadiness especially under strain.

【Phlegm】 suggests an apathy of mind or sluggishness of temperament that results from a physical condition rather than from discipline or self-control; it therefore suggests even greater imperturbability and insensitiveness than any of the preceding terms.