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.