a point at which a decisive change takes place; critical point; crisis.

a point at which something changes direction, esp. a high or low point on a graph.

Surveying . a point temporarily located and marked in order to establish the elevation or position of a surveying instrument at a new station.

noun

axis , change , climacteric , climax , contingency , crisis , critical mass , critical moment , critical period , critical point , crossing , crossroads , crucial moment , crucial occurrence , crucial period , crunch , crux , culmination , cusp , decisive moment , defining moment , development , emergency , exigency , high noon , hinge , juncture , kairotic moment , moment of truth , moment of truth * , nexus , pass , peak , peripeteia , pinch , pivot , pivotal moment , point of no return , race against time , rising action , shift , strait , transition , turn of the tide , twist , when push comes to shove , zero hour , zero hour * , crossroad , exigence , head