What is the literal meaning of the word 'Upanishad'?

The main theme of Upanishads is –

Which of the following is the correct order of the Vedic literature?

In which Upanishad is the famous dialogue of Yagnavalkya and Gargi mentioned?

During the Rigvedic Period word ‘Nishka’ was used for a Jewellery but in later period it was used for the meaning of :

What is the total number of Sanskaras?