In one word 'a Wave'.
In trigonometry you learn of the Sine, Cosine and Tangent function'.
Taking the 'Sine' function and graphing it between 0 , & 360 degrees, it produces a 'wave'. If you continue to angular measurement in either direction , it produces a series of waves.
The word 'Sine' from Latin , means 'curve' or 'Wave'. . The '-oidal' bit means 'like'. So the graph is producing a continuous cure or wave, ,which is named 'sinusoidal.
'Cosine' means 'Complementary Wave/curve'.