In a manner that is secondary or indirect; as a side effect or consequence rather than as the main purpose.
From Medieval Latin 'collateralis' (side by side) with English adverbial suffix '-ly'. The word carries the original Latin sense of being "on the side of" the main action.
When you study history to understand the present, you're learning world events collaterally—it's not your main goal, but it happens as a side benefit of your primary study.
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