Are you talking about Edward O. Wilson's idea to devote half of the earth to nature?
If so then "half of Earth; Our Planet's fight for life", is mainly a book written by Edward Wilson who proposes the idea of preserving biodiversity for generations [to come] by maintaining half of the Earth as a "human-free natural resource". The book can be found on Amazon.
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If you're talking about what the name of the half of earth on a globe is called then, the top hemisphere is known as the "Northern Hemisphere" while the bottom hemisphere is known as "Southern Hemisphere". The line dividing both, at 0 degrees is known as the "Equator".
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If you are talking about something completely different, like half-life, then I suppose that expanding on your question would be a tad bit more helpful.
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Hope I helped.