We've historically been somewhat inconsistent in how we refer to the
type of object defined by "variable" blocks in configuration. Parts of
our documentation refer to them as "input variables" or just "variables",
while our implementation refers to them as "user variables".
Since Terraform Registry is now also referring to these as "Inputs", here
we standardize on "Input Variable" as the fully-qualified name for this
concept, with "variable" being a shorthand for this where context is
obvious. Outside of this context, anything that can be referred to in
an interpolation expression is generically known as a "variable", with
Input Variables being just one kind, specified by the "var." prefix.
While this terminology shift is not critical yet, it will become more
important as we start to document the new version of the configuration
language so we can use the generic meaning of "variable" there.
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