Intuitively, a succinct data structure is one whose space usage equals the space needed to write out the data, plus something that grows more slowly than that. If you’re familiar with little-o notation, a succinct data structure is one whose space usage is X + o(X), where X is the number of bits needed to write out the data itself. - SO / HN

see also

• Fast, Small, Simple Rank/Select on Bitmaps (pdf)
• Gonzalo Navarro