Sigma Level (Z) - is a representation of the process capability in terms of opportunities and defects. It refers to number of defects per million opportunities (DPMO). Higher the sigma level, lower the number of defects
1σ = 691,462 DPMO
2σ = 308,538 DPMO
3σ = 66,807 DPMO
4σ = 6,210 DPMO
5σ = 233 DPMO
6σ = 3.4 DPMO
Also, by definition, Sigma Level (Z) is the number of standard deviations that can fit between the mean and the specification limit. E.g. for a process operating at three sigma level, three standard deviations can be fitted between the process mean and the specification limit.
An application oriented question on the topic along with responses can be seen below. The best answer was provided by Arunesh Ramalingam on 9th November 2017.