An outliner is software for writing in outline form, where each piece of information sits on its own bullet and bullets nest under one another as parents and children.
Logseq's introduction to the form starts from the list: "If you've ever written a list, you've written an outline." In what the post calls outliner-speak, the relationship between an item and a subitem is a parent-child relationship. Expanding an outline means a parent item shows its children; collapsing hides the children so only the parent items show; and clicking a bullet zooms in on that item and all of its subitems.
📝Logseq describes itself as a networked outliner in which each bullet is a block, best compared to a paragraph, that can connect to any other block or to pages. To show one block from a branch elsewhere without the rest of the branch, the post points to a 📝block reference.
Workflowy's homepage says "A single bullet list does it all." Its getting-started guide explains that pressing Tab indents an item, "making it a child of the one above," and that clicking any bullet makes it "your whole world," hiding everything outside it until a breadcrumb leads back. Dynalist calls itself an outlining app.