The plus sign (
+) is for selecting the next sibling.
Is there an equivalent for the previous sibling?
No, there is no “previous sibling” selector.
On a related note,
~ is for general successor sibling (meaning the element comes after this one, but not necessarily immediately after) and is a CSS3 selector.
+ is for next sibling and is CSS2.1.
See Adjacent sibling combinator from Selectors Level 3 and 5.7 Adjacent sibling selectors from Cascading Style Sheets Level 2 Revision 1 (CSS 2.1) Specification.