To find an in-depth definition of evidence one could look at how it is described in various on-lines encyclopedias. Here are some.
Wikipedia describes evidence in the following way:
Evidence, broadly construed, is anything presented in support of an assertion. This support may be strong or weak. The strongest type of evidence is that which provides direct proof of the truth of an assertion. At the other extreme is evidence that is merely consistent with an assertion but does not rule out other, contradictory assertions, as in circumstantial evidence.
What this tells us is that an "assertion" comes first. Evidence is something that supports an assertion. Also some evidence is strong and some evidence is weak.
Let's move to the Internet Encyclopedia of Philosophy. Victor DiFate describes evidence as:
The concept of evidence is crucial to epistemology and the philosophy of science. In epistemology, evidence is often taken to be relevant to justified belief, where the latter, in turn, is typically thought to be necessary for knowledge. Arguably, then, an understanding of evidence is vital for appreciating the two dominant objects of epistemological concern, namely, knowledge and justified belief. In the philosophy of science, evidence is taken to be what confirms or refutes scientific theories, and thereby constitutes our grounds for rationally deciding between competing pictures of the world. In view of this, an understanding of evidence would be indispensable for comprehending the proper functioning of the scientific enterprise.
Here we learn that evidence is relevant to "justified belief" and "knowledge" which "epistemology" studies. Evidence can also be used to confirm or refute scientific theories. A study of evidence from the perspective of the scientific enterprise would involve the "philosophy of science".
Lastly, let's consider what the Stanford Encyclopedia of Philosophy has to say about evidence. Thomas Kelly claims there is a "tension" about what evidence means to different people::
Moreover, it is not simply that the accounts of evidence that have been advanced by philosophers stand in at least some prima facie tension with much that is said and thought about evidence outside of philosophy. As even the cursory survey offered above makes clear, philosophers themselves have offered quite divergent theories of what sorts of things are eligible to serve as evidence.
This brings us to the OP's question: What is evidence?
There is some general agreement about what evidence is based on the first two references. It is what justifies belief and provides knowledge. It is what allows us to confirm or refute scientific theories.
However, there is also tension between the different ways people use the word evidence and as Kelly further remarks, "it is far from obvious that any one thing could play all of the diverse roles that evidence has at various times been expected to play."
DiFate, V. "Evidence" Internet Encyclopedia of Philosophy https://www.iep.utm.edu/evidence/
"Evidence", Wikipedia https://en.wikipedia.org/wiki/Evidence
Kelly, Thomas, "Evidence", The Stanford Encyclopedia of Philosophy (Winter 2016 Edition), Edward N. Zalta (ed.), URL = https://plato.stanford.edu/archives/win2016/entries/evidence/.