means the capacity to grasp the true, often hidden nature of something, or the clear understanding that arrives when you do.

from A plain English build: "in" + "sight," literally inner seeing — sight turned inward rather than outward. It dates to Middle English, where it carried the sense of inward vision or wisdom, and likely owes its shape to similar Germanic compounds (compare Middle Dutch and German words built the same way). The metaphor is older than the word: seeing as a stand-in for knowing runs deep through language, so to have insight is to have light fall on what was dark within.