Final answer:
The 'border-collapse' CSS property determines if the borders of adjacent cells are separated or collapsed into a single border.
Step-by-step explanation:
The question refers to the CSS property that determines whether there is space between the borders of adjacent cells or the borders are collapsed to a single border between the cells. In CSS, this is controlled by the 'border-collapse' property. When set to 'collapse', borders are combined into a single border. Conversely, when set to 'separate', space is maintained between the borders of adjacent cells. This property is particularly useful in the styling of HTML tables to achieve the desired look.