A complex number
is associated with the sinusoid. F is called the phasor of f(t). The sinusoid is said to be represented by f(t) in the Time Domain and by F in the Frequency Domain.
This figure shows a phasor, F, in the Frequency Domain and corresponding sinusoid, f(t), in the Time Domain. Left click in the Frequency Domain with mouse to change the phasor. The Time Domain will change to show the sinusoid corresponding to the new phasor. (Each phasor is labeled using its real and imaginary parts: (real part, imaginary part).)