1.
private sector; non-governmental; civilian
The sphere of society outside the government, including private businesses, organizations, and ordinary citizens. Used as a noun or as a modifier (民間の) to contrast with public/governmental entities.
2.
folk; popular; among the people
Traditions, beliefs, remedies, or practices that originate from and are transmitted among ordinary people rather than through official or academic channels.
Usage
民間 is most commonly used as a modifier (民間の or in compound form) to contrast with government or official entities. Sense 1 (private sector) is the more frequent usage in modern Japanese, especially in business and political contexts. Sense 2 (folk) appears in cultural and medical contexts.