Does anyone know the legal meaning of "collusion"?
"Collusion" has no meaning in U.S. federal law. It's a catch-all term used by the press and President Trump to refer to whether persons associated with the 2016 Trump campaign participated in the efforts by the Russian government to influence the outcome of the presidential election.
The federal crimes that the special counsel reportedly is investigating include "active measures" espionage by Russia and possibly other governments (i.e., attempts to influence the outcome of the 2016 presidential election); various other computer crimes committed in the course of attempts to influence the election (e.g., identity theft); making illegal campaign contributions (those that exceeded the statutory maxima or were funded by foreign donors); obstruction of justice; making false statements to federal agents (e.g., investigators from the FBI); conspiracy to commit these crimes—and, apparently, a number of personal financial crimes and attempts to evade paying U.S. taxes that the investigators turned up in the course of pursuing the primary objectives of their inquiry, some of which come within the purview of the special counsel and others which have been referred to the U.S. attorneys' offices with jurisdiction over them.
Some of these areas of inquiry are known from the guilty pleas the special counsel has already obtained from targets of these investigations, some from public court filings by the special counsel, and some from "leaks" to the press that appear to have originated with lawyers for various defendants or witnesses. This last group of sources obviously may be self-interested in the information they make public, and their revelations should be considered with some skepticism since it is all but impossible to verify them; in my nearly 50 years in Washington, some of them spent as a news reporter and analyst (I'm also a lawyer), I don't think there has ever been a federal investigation whose existence was known to the public that has been conducted in such secrecy. Robert Mueller really has kept the lid on.
So far, the only crimes that can reasonably be said to have been committed are those to which the defendants have pleaded guilty. The special counsel has also filed some other charges which are being contested, and which will have to be resolved in court or perhaps by future guilty pleas. However, the court warrants that have been issued to the special counsel to conduct searches and seizures of information—some of them "no-knock" warrants where the subject or the subject's lawyer was not given notice of the search prior to its initiation, a rarity in white-collar crime investigations—strongly suggests that there will be additional charges to follow.