The magnitude of the waveform has taken at any point of time is called instantaneous value. And the instantaneous value of the alternating quantity is designated by a lower case letter small e for emf, small v for voltage and small i for current.
The instantaneous values of Waveform can be determined either from the curve which you can get it from oscilloscope or from an equation of the alternating quantity. For example, the instantaneous values of emf represented by the curve shown in Fig. 1 at 0, pi/2, pi and 3pi/2 are zero, +Emax, zero and -Emax respectively.
Refer the diagram:
The value of i1, i2, i3…in gives the value of the sine wave at that instant of time.