The term 'nail down' is a phrase used to describe the act of making something certain or definite. The crossword solutions for this phrase include ICE (3 letters), CLINCH (6 letters), ENSURE (6 letters), AFFIRM (6 letters), ASSURE (6 letters), SECURE (6 letters), and TRANSFIX (8 letters). Each of these words can be used to describe the act of making something certain or definite, with ICE being the most concise and TRANSFIX being the most descriptive.