What does LINE_4, LINE_8 mean?

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If I zoom in on small circles drawn with the following C++ code:

const int radius = 4;
const int thickness = 1;
cv::circle(mat, p1, radius, black, thickness, cv::LINE_4);
cv::circle(mat, p2, radius, black, thickness, cv::LINE_8);
cv::circle(mat, p3, radius, black, thickness, cv::LINE_AA);

I see this:

The anti-alias label I understand. But what exactly does LINE_4 and LINE_8 mean?