Final answer:
The 'difficult' temperament describes children who are less regular with body functions and slower to stabilize eating and sleeping routines, often displaying negative emotions and adaptation challenges.
Step-by-step explanation:
The term used to describe the temperament of children who are less regular with their body functions and slower to develop stable eating and sleeping patterns is 'difficult'. According to researchers Thomas and Chess, children with a difficult temperament may show negative emotions and have a harder time adapting to changes and regulating their emotions. These traits can make it challenging for parents and caregivers to establish routines including regular eating and sleeping patterns. Environmental factors and maturation can also affect how these personality traits express themselves in children.