Which specific Denavit-Hartenberg parameter represents the shortest distance between the Z-axes of two consecutive robot joints?

The Denavit-Hartenberg parameter that represents the shortest distance between the Z-axes of two consecutive robot joints is the link length, denoted by the symbol 'a' or sometimes 'ai'. In the Denavit-Hartenberg convention, a robot arm is modeled as a series of joints and links. The Z-axis of each joint represents the axis of rotation o....