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Deforestation is a major driver of climate change and the major driver of biodiversity loss. Yet the essential baseline for monitoring forest cover—the global area of forests—remains uncertain despite rapid technological advances and international consensus on conserving target extents of ecosystems. Previous satellite-based estimates of global forest area range from 32.1  ×  106km2to 41.4  ×  106km2. Here, we show that the major reason underlying this discrepancy is ambiguity in the term ‘forest’. Each of the >800 official definitions that are capable of satellite measurement relies on a criterion of percentage tree cover. This criterion may range from >10% to >30% cover under the United Nations Framework Convention on Climate Change. Applying the range to the first global, high-resolution map of percentage tree cover reveals a discrepancy of 19.3  ×  106km2, some 13% of Earth’s land area. The discrepancy within the tropics alone involves a difference of 45.2 Gt C of biomass, valued at US$1 trillion. To more effectively link science and policy to ecosystems, we must now refine forest monitoring, reporting and verification to focus on ecological measurements that are more directly relevant to ecosystem function, to biomass and carbon, and to climate and biodiversity.