An uncodified constitution is a type of constitution where the fundamental rules often take the form of customs, usage, precedent and a variety of statutes and legal instruments. An uncodified constitution has the advantages of elasticity, adaptability and resilience. A significant disadvantage, however, is that controversies may arise due to different understandings of the usages and customs which form the fundamental provisions of the constitution.
Israel is one of only three democracies in the world without a codified constitution. The other two are New Zealand and Britain.
Source: https://en.wikipedia.org/wiki/Uncodified_constitution, https://www.buzzfeed.com/rethinkisrael/facts-about-israel-that-will-surprise-you?utm_term=.kt9wbRzkB#.if919BJXd