Aunit circle is a circle with a radius of 1 that is useful in trigonometry. Angles are measured starting from the positive x-axis in quadrant 1 and continuing counterclockwise. For the ACT, you should definitely know the radian measures of the axes and in which quadrants the trig functions are positive or negative.
Important to know! The mnemonic All Students Take Calculus can help you remember where functions are positive: Q1: all, Q2: sine, Q3: tangent, Q4: cosine.