Outside the US, the legal definition is “available to the public”. This means any step which puts the information within reach of someone who is not under your control. So selling an article on the open market allows any buyer to dismantle the product and look at it. Publishing something on the web means anyone can read it, and archiving services can keep a copy of it. Speaking at a conference means that anyone can listen (or read the proceedings). Placing a book in a library means anyone can read it, even if none actually do. And so on.
Something is not disclosed for the purposes of patent law if it is not published in any way, or if the only people who the inventor tells are bound by some duty of confidentiality. This can be implicit (such as in the case of co-workers or professional advisors), or it can be explicit (such as with an outside collaborator who signs a non-disclosure agreement).
Different rules apply in the US, generally allowing more freedom. However, if you are considering protecting the idea outside the US, then you need to stay within the more restrictive rules of the other countries in which you might be filing.