Definition of Paired T-Test:
The two-sample t-test is used to determine if two population means are equal. The data may either be paired or not paired.
For a paired t-test, the data is dependent (i.e., there is a one-to-one correspondence between the values in the two samples).
For example, same subject measured before and after a process change, or same subject measured at different times.
For unpaired t test, the sample sizes for the two samples may or may not be equal.