In English:
Null: without value, effect, consequence, or significance.
In Programming:
Null: used in computer programming for an uninitialized,
undefined, empty, or meaningless value
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Elaborating on that idea, "Undefined" means that a variable has not been previously defined... so what does defined mean?
In English:
Defined: explained or
set the meaning of.
In Programming:
Defined: value has been stored into a variable.
Keep defining words and using google.
That's how I taught myself how to program.