Concept:
A mouldboard plough consists of parts such as share, mouldboard, landside, frog, and frame.
Step 1: The share is the cutting part of the plough.
Step 2: The mouldboard turns and inverts the soil slice.
Step 3: The frog is the central part to which the share, mouldboard and landside are attached.
Step 4: The edge of the share forming the joint between the mouldboard and the share on the frog is called the cleavage edge.
Step 5: Hence, the correct answer is cleavage edge.
\[
\boxed{\text{Cleavage edge}}
\]