A succinct definition of intelligence

Intelligence  is the operator brackets IF-THEN-ELSE. 

These brackets are perhaps the most succinct expression of the essence of decision making. Their implementation details do not matter. For instance, they do not have to be implemented explicitly programmatically. In order for an arbitrary system to conform to the definition of intelligence, all that is necessary is that this system should execute the brackets.

The simplest example is probably a thermal bimetallic refrigerator relay (fig. 1).

Fig.1. Various types of thermal defence relay.

A far more complex example is so called bacterial chemotaxis (fig.2). It is a molecular effect whereby a unicellular organism moves along chemical gradients: towards attractants such as food (positive chemotaxis) or away from repellents, i.e. harmful substances (negative chemotaxis).

Fig.2. Bacterial chemotaxis. Source: wikipedia.