Siblings share 50% of their DNA. Then scientists say we have 1-4% Neanderthal DNA, but say we are 99% related to chimps. Then there is the fact that we share 60% of our DNA with bananas.
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There is no single meaning. People are using the term in different contexts with different meanings. That’s why it seems confusing or contradictory – because they are talking about different things.
For example, there’s a difference between genetic similarity between *people* and *species*. When talking about people, you’re looking at a specific “snapshot” of DNA; when talking about species, you’re looking at a group of “snapshots” of DNA.
As an analogy – it’s like comparing the English words “horse” and “house” (people) vs. comparing the English alphabet with the French alphabet (species).
Further, there are different ways to compare between species. There is a difference between looking at “absolute” similarity – how many genes are shared – and “relative” similarity, which can be measured in various ways. For example, you could measure something like “how many genes *first appeared* in species X and *then* appeared in species Y” as a way to track how the species interbred; this measure would ignore all the things that are already similar in the species’ common lineage before they split into X and Y. There are other relative measures you could take as well.
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