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If the definition follows immediately, I'd use "as follows:" with a colon.
This is defined as follows:
(definition follows immediately)
If the definition doesn't follow immediately, but after something else, I'd use "below." with a full stop.
This is defined below.
(Some further discussion)
(Definition is reached at some point)
This isn't grammar; it's a matter of style. But "X is defined as follows" indicates that the definition follows (immediately); "X is defined below" indicates that you will find it if you carry on reading.
"Below" could work for both.
But using a colon indicates "follows now" and a full-stop indicates "follows later".