What is a stellium?
Three or more planets in one sign or one house. It is where the weight of a chart sits: that style, or that area of life, is louder than everything else, and any transit through it lands on several planets at once. A person with a stellium is often described by it more than by their Sun sign.
Where astrologers disagree
Whether three counts or four, and whether the nodes and angles are included, are conventions. Here it is three planets, with the nodes left out.
Where you see this in OrbHeaven
Your own chart is calculated from your birth details, and this definition is written out again there with your numbers in it rather than in the abstract. The chart, the glossary and thirty days of dated timeline are free, and nothing here is charged per question.
About a minute. Birth time optional, and no account until after you have seen it.
The chart
What the picture is, and what the numbers on it are measuring.