Question

the term natural rate of unemployment refers to

Answer 1

Natural rate of unemployment is the minimum unemployment rate resulting from real or voluntary economic reasons. Natural unemployment reflects the number of people that are unemployed due to the structure of the labor force, such as those replaced by technology or those who lack certain skills to gain employment.
It also includes people who are voluntarily unemployed- people who are capable of employment but are unemployed because of there own choice or desire.

The combined unemployment rate due to these two above forces is the natural rate of unemployment. It usually is the minimum rate of unemployment that will be there too even in ideal case, no matter what policies the government enacts.