rea ‧ son ‧ ing /ˈriːz ə nɪŋ/ BrE AmE noun [uncountable]
[ Word Family: noun : ↑ reason , ↑ reasoning , ↑ reasonableness ; adjective : ↑ reasonable ≠ ↑ unreasonable , ↑ reasoned ; verb : ↑ reason ; adverb : ↑ reasonably ≠ ↑ unreasonably ]
a process of thinking carefully about something in order to make a judgment
scientific/logical/legal reasoning
reasoning behind
What is the reasoning behind this decision?