'Nisseem' shabd mein kaun si sandhi hai? (Which sandhi is in the word 'Nisseem'?)

The word 'Nisseem' is formed by joining 'Nih' + 'Seem' (निः + सीम = निस्सीम), meaning 'boundless/limitless'. Since the first word ends in 'visarg' (ः) and joins with the consonant 's' of seem, transforming the visarg into 's', this is a classic example of Visarg Sandhi. It is not Swar sandhi (vowel + vowel) or Vyanjan sandhi (consonant + consonant/vowel where neither is visarg). Deergh sandhi is a sub-type of swar sandhi which doesn't apply here.