Dimension is a noun.
A measurable extent of a particular kind, such as length, breadth, depth, or height:
Length is a dimension in this system.
Transforming a noun into an adjective with -al denotes relating to or kind of:
The noun dimension + -al produces the adjective dimensional:
relating to dimension.
The Y axis is a dimensional reference for length in the system.
Forming a noun with -ity denotes a quality or condition:
The adjective dimensional + -ity produces the noun dimensionality:
the condition of relating to dimension.
An additional axis changes the dimensionality in the system.
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The noun dimension refers to the actual thing.
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The adjective dimensional refers to relating to the actual thing.
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The noun dimensionality refers to the condition of relating to the actual thing.