Lachrymal Glands -
The small glands which are responsible for tear production. They often become active as hypnosis deepens.
Lalophobia -
The irrational fear of speaking.
Latent Time -
The time between stimulus and response. A period often extended as hypnosis deepens.
Lethargy -
Early term coined by J. M. Charcot for the light or early stage of hypnosis.
Levitation -
Where a limb is caused to rise by suggestion. Often used in hypnotic induction and deepening. Useful where the ensuing therapy employs partial dissociation or glove anaesthesia.
Liminal -
The threshold. 
Liminal Sensitivity -
The threshold of sensation. The minimum stimulus required to cause sensation.
Hence, Subliminal - beneath the threshold of sensation.
Locus of Control -
The place where a person experiences the controlling influence in their life to emanate from. A person with an internal locus of control feels that they control their life from within themselves and are responsible for all things that happen to them. A person with an external locus of control feels that their life is governed by forces external to them over which they have no real influence.
Lucid Dream -
A dream in which the participant realizes that they are dreaming. Can be evoked by post hypnotic suggestion.