A term used by Montessori to describe a critical time during human development when young children are neurologically ready to learn a specific skill or ability.

During this time, an individual becomes particularly sensitive to stimuli that promote the specific developmental acquisition. 

Montessori observed that during a sensitive period, children are intensely interested in tasks that are related to the particular area of development, sometimes to the exclusion of everything else.

For example, children in the sensitive period for numbers (4- 4 1/2 years) will be drawn to work with the Maths Materials. 

When children choose such activities they become absorbed in deep concentration.