Musubi (ki no musubi) was one of the terms I heard early in my education. I think this term is probably what many consider "aiki". In aikido, this would like as #1 for me, since I would argue without the "aiki" we are doing jutsu.
I actually prefer to think about kokyu as the expansion and contraction of musubi. I'd probably say the pressure of suki is #3 from me. That is, the concept that suki is the expression of openings created from a gap in musubi.
If a line of pressure/connection/musubi/ground path/(insert term-du-jour here) through the structure loses its continuity, the one of two things happens:
1) It collapses -- and progressively -- if there is sufficient dynamic applied to the discontinuity; or
2) It creates a suki/disjoint/lever/suit crease/(etc.) in the structure that requires an internal counter-lever to compensate. This has the effect of isolating any remaining power or structural reserve behind that discontinuity from being applied or brought to bear.
When we in aikido do various "lock-like" maneuvers (nikkyo/sankyo/yonkyo/shiho/kotegaeshi) and pins they are not for the purposes of locking and pnning per se, but to feel where and how these disjunctions occur or are created and how to deal with them when we feel them occurring -- on both sides of the training.