Full Idea
The system K has no frame conditions imposed on its accessibility relation.
Gist of Idea
The system K has no accessibility conditions
Source
M Fitting/R Mendelsohn (First-Order Modal Logic [1998], 1.8)
Book Reference
Fitting,M/Mendelsohn,R: 'First-Order Modal Logic' [Synthese 1998], p.19
A Reaction
The system is named K in honour of Saul Kripke.