I absolutely loved this question. Of course, we can all think of a handful of words like thingy, that simply act as a stand-in word for the word we are thinking of but cannot, at present, grasp.
The 1960 Dictionary of American slang uses the term kadigin for placeholder words, defining it as a synonym for thingamajig.
Dr. Richard Nordquist, Professor Emeritus of English, writes in Crossing Boundaries: Studies in English Language, Literature, and Culture in a Global Environment, originally published in 2009:
The linguistic term for such peculiar sounding words as "thingamajig" and "whatchamacallit" is placeholder, or, less formally, tongue-tipper or kadigin: a word used to signal that a speaker does not know or cannot remember a more precise word for something.
William Safire, author, presidential speechwriter, and writer of The New York Time's column "On Language" addresses popular etymology in his incredibly popular, and entertaining, column. The focus of his January 9, 2005 piece, "Whosit's Whatchamacallit" is all about your topic:
"We are now into the creative world of "tongue-tippers," terms used in place of words on the tip of the speaker's tongue but just beyond linguistic reach."
In the article, Safire provides fascinating history and etymology of various tongue-tippers. We also learn that English may not be able to lay claim to all tounge-tippers:
"British English also has its words for the unremembered objects. In 1962, The Sunday Times explained that "'ujah' . . . was used as widely and as indiscriminately as 'gimmick' and 'gadget' are used now." It was usually spelled oojah and was thought to be of Hindustani origin."
I hope you enjoy reading the entirety of Safire's column. Between placeholder, kadigin and tongue-tipper, the last one in my opinion is the best. I think that it's important to have a memorable word when trying to remember the name for the group of words that we use when we just can't seem to remember the actual word.
Your question is a little bit complicated to me, but do you mean a self-referencing word?
If so, then it is usually called an autological word or an autonym.
Autogram - A sentence that describes itself in the sense of providing an inventory of its own characters.
This sentence has five words.
This sentence contains nine syllables.
Reflexive (sentence)
A sentence where the subject and object are the same (Sources: Reflexive verb, pronoun).
The man washed himself.
Hofstadster's Law
Hofstadter's law is a self-referential time-related adage, coined by Douglas Hofstadter and named after him.
Any task you're planning to complete will always take longer than expected - even when Hofstadter's law is taken into account. Even if you know a project will overrun, and build that knowledge into your planning, it'll simply overrun your new estimated finish time, too, Hofstadter says.
(Source: The Guardian)
Post hoc ergo propter hoc
Latin for after this, therefore because of this. It means a logical fallacy (of the questionable cause variety) that states "Since event Y followed event X, event Y must have been caused by event X." Sometimes shortened post hoc.
The server was working fine until you started working here. Therefore, you must be the reason why it's broken.
Recursion
Recursion is the process of repeating items in a self-similar way. (Warning: Has many applications outside of language as well.)
To understand recursion, you must understand recursion.
Best Answer
You may be looking for metonymy.
If you're looking for other examples, governments are often referred to like this—at least, Westminster for the UK parliament, and Washington for the US government. (In fact, looking up Westminster on Wikipedia was how I found metonym.)