Intelligence is quite easy to define, it is the ability to comprehend a situation and use the knowledge or skills at your disposal to determine the best course of action from your comprehension of the problem. Basically cognitive reasoning.
It is the complexity of this intellectual pursuit that often results in people thinking they are intelligent, without having the knowledge and skills at their disposal to look at complex issues, and simplify them or outright reject the knowledge we have collected and ignore that in favour of an ideology, that issue is one of the main problems facing the world today.
It is the difference between using objective scientific facts and subjective beliefs.
The best analogy I have heard is "science flies you to the moon" this is an objective fact and "religion flies you into buildings" based on subjective beliefs.
Which circles very nicely back to the comprehension aspect of defining intelligence as it is the cognitive reasoning function of intelligence that determines if someone is actually intelligent or not. This is also where bias plays a part as ideologically trapped people forego the cognitive function for the sake of their ideology.