In a manner relating to or expressing cause and effect; with respect to causation.
From 'causal' (adjective, relating to cause) plus the adverbial suffix '-ly'. The chain extends back to Latin 'causa' through English 'cause' and its derivatives.
When scientists say something happened 'causally connected,' they're claiming one thing definitely caused another—it's fighting words in research because proving true causality is way harder than just finding two things that happen together.
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