I call line-breeding deliberately breeding back to a
specific ancestor (or ancestors) that had specific features that one wants to perpetuate, was an overall excellent example of the breed and preferably was known to be prepotent. Thus I'd say I line breed on Shenval Druid who was top stud for three years despite being very lightly used (and despite never being shown as he lost the end of his tail in a puppyhood accident

)
Mating related stock, eg with mutual grand-parents, without trying to go back to any specific ancestor is just what I'd call a degree of inbreeding.
But then there are no hard and fast definitions in this :)