Definition: The first stage of Piaget's cognitive-development theory. Occurs from birth to two years - a time where a persons knowledge is based upon physical interactions and experiences they have.

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Definition: a meaningful morphological unit of a language that cannot be further divided (e.g. in, come, -ing, forming incoming ).

Definition: The preoperational stage is the second stage in Piaget's theory of cognitive development. This stage begins around age two and lasts until approximately age seven. During this period, children are thinking at a symbolic level but are not yet using cognitive operations.

Definition: The concrete operational stage is the third stage in Piaget's theory of cognitive development. Kids at this age become more logical about concrete and specific things, but they still struggle with abstract ideas.

Definition: The formal operational stage is the fourth and final stage of Jean Piaget's theory of cognitive development. It begins at approximately age 12 and lasts into adulthood. At this point in development, thinking becomes much more sophisticated and advanced.