In the illustration of a cell undergoing cytokinesis, the slight indentation indicated by the letter X is the cleavage .

The slight indentation indicated by the letter X in the illustration of a cell undergoing cytokinesis is the cleavage furrow.

Step 1: Understand the question - The question is asking about a specific part of a cell that is labeled as 'X' in an illustration. This part is described as a 'slight indentation' in a cell that is undergoing cytokinesis.

Step 2: Identify the key terms - The key terms in this question are 'cell', 'cytokinesis', 'indentation', and 'X'.

Step 3: Recall relevant information - Cytokinesis is the process where a single cell divides into two separate cells. During this process, an indentation known as the cleavage furrow forms at the equator of the cell.

Step 4: Apply the information - The 'slight indentation' referred to in the question is the cleavage furrow. This is because the cleavage furrow is the only structure that forms as an indentation during cytokinesis.

Step 5: Formulate the answer - Therefore, the letter 'X' in the illustration of the cell undergoing cytokinesis is indicating the cleavage furrow.