The crossword solutions for the term 'Central Difficulty' are RUB, which stands for a rubbing or friction. Rubbing is the process of two surfaces moving against each other, resulting in friction and heat. This is a common phenomenon that can be seen in everyday life, such as when two objects RUB together or when a person rubs their hands together to generate heat. Rubbing can also cause wear and tear on materials, making it a central difficulty in many engineering and manufacturing processes.