Where is previous_posts_link defined?

Hello, I am updating a theme (newspaper-2b-10) to WP21. The theme uses the tags previous_posts_link and next_posts_link. Tried to find those tags in the codex but they seem not to be there. Instead, I found previous_post_link and next_post_link, ‘post’ instead of ‘posts’. Where are those tags described.

Thanks,

GS