preloader
Noun
Meaning
Agent noun of preload; one who, or that which, preloads.
(web development, historical) In Flash animations, the loading screen that stalls for time while resources are downloaded.
Source: en.wiktionary.orgExamples
However, with the browser’s preloader, only resources which are declared in the HTML document are preloaded.
And so especially for the top level component, it's very common to actually put the, really a preloader right in there in the middle.