Solid or Rigid Training: This basic level helps one to practice stable hip movement, "ki" extension, breathing, coordinated body movement, and other basic principles.  At this level of training one moves against strength and power to discover the proper internal and external structural alignment to facilitate minimal effort in movement.  Often this level is compared to the hardness of a Diamond symbolically.

Flexible: At this intermediate level one begins to add timing and more movement to the basic level of training.  Impending attacks are responded to with a minimum of effort and a maximum of efficiency, the result of proper timing and movement.  Willow or bamboo is representative of this level.

Flowing: In this further elevated level, the principle of 'leading when one is attacked ' is learned.  This is accomplished by 'reading' the intention of the attacker(s), before physical contact is made.  Rhythm, timing, and body movement (tai sabaki) are paramount to the efficient application of Aikido techniques at this level.  The natural element of water, is symbolic of this principle.

Takemusu Aiki: "Divinely inspired Aiki".  At this advanced level, techniques are created spontaneously to suit the circumstances moment to moment.  In its truly elegant manifestation, Takemusu Aiki could be expressed as "effortless perfection" or "formlessness".
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Riai: This is a concept whose principle understanding stresses that Aikido techniques, and movements remain the same whether one is armed or unarmed, against one or multiple assailants.