The word "intelligence" comes from Latin "intelligo" which means "I understand". So, the basic meaning of intelligence is the ability to understand, or to "get it", so to speak. Everyone has it, but not to the same degree (hence IQ). People who get it faster are considered more intelligent than others.
It may be interesting to analyze the word further. "Intelligo" is actually "inter-ligo", that is, "I tie between". That would indicate that intelligence is the ability of seeing relationships among things and ideas, the ability "to put it all together."