Tension is a type of Strain. Strain is a type of Tighten. Tighten is etymologically related to Tight. 
Slight is related to Mere. Mere is a synonym of Simple. Simple is a synonym of Plain. 
Tension --IsA--> Strain --IsA--> Tighten --EtymologicallyRelatedTo--> Tight 
Slight <--RelatedTo--> Mere <--Synonym--> Simple <--Synonym--> Plain 