With UI design terms like these, there can be many popular terms for the same thing. Companies will purposely use unique terminology for business reasons and legal reasons. People will misuse terminology because it's technical. Menu is definitely a technical word but these terms are not.
These are very generic terms though. For example, every menu that expands vertically is a dropdown. Material Design Guidelines rarely uses the term dropdown. Dropdown menus are simply referred to as menus: https://material.io/guidelines/components/menus.html#menus-simple-menus
Look to popular design guidelines to find or validate UI terminology. The terminology used by design guidelines is itself designed by designers and developers working together.
In general, there are two types of menus in UI. The first type of menu is a list of commands. Commands make the application do something. This is the original core programming UI. The second type of menu is a only a selection tool. The menu has only one command and the items in menu are just options for that command. A navigation menu is this type of menu.
There are differences between the terms you're asking about. Dropdown Menu is a desktop design. Popover menu is a touch design. Fly-out menu is a desktop dropdown menu sub-menu.
Dropdown is the original description given to expanding text menus on desktop. Dropdown refers to the visual effect of the menu opening. So all menus dropdown but the term is generally used for keyboard and mouse menus (desktop) as opposed to touch menus.
Popover refers to the visual effect of the menu appearing on top of everything visually. So all menus popover. The term is used for touch menus.
Fly-out refers to opening horizontally as opposed to vertically. The term is used for desktop dropdown sub-menus. Fly-outs are dropdowns.
Here are examples of dropdown and popover from Google's Material Design Guidelines and Apple's Human Interaction Guidelines for Mac OS and iOS. Also shows how Apple uses "popup" for their Mac OS as opposed to "popover".
