Attractor
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According to Wikipedia:
In the mathematical field of dynamical systems, an attractor is a set of numerical values toward which a system tends to evolve, for a wide variety of starting conditions of the system.[1] System values that get close enough to the attractor values remain close even if slightly disturbed. In finite-dimensional systems, the evolving variable may be represented algebraically as an n-dimensional vector. The attractor is a region in n-dimensional space. In physical systems, the n dimensions may be, for example, two or three positional coordinates for each of one or more physical entities; in economic systems, they may be separate variables such as the inflation rate and the unemployment rate. Wikipedia
In the context of 2D and 3D geometric modeling in Rhino and Grasshopper, an attractor can be considered as a geometric object that influences other geometric objects. This influence can be designed to impact attributes of the influenced object or set of objects, such as radius, position, color, thickness etc.
Many Attractor tutorials for Grasshopper, including the Grasshopper Primer by Mode Lab, start by creating a hexagonal grid using the vector > grid > hexagonal, where the center-point of each grid is used to create a circle using: curve > primitive > circle: CNR (circle, normal, radius).
If a point object is created in Rhino, it can be referenced in a Grasshopper definition using the parameter > geometry> point object. Then it can be configured to act as an attractor to influence the circles contained in the each hexagonal-grid cell. In the image below, the attractor point is used to create a vector between each circle's center and the point. This vector is used as an input to the normal vector for the circle component, so all circles are now oriented towards the attractor point. In addition, an offset curve component, and a freeform boundary surface is created from this offset curve.
The image below shows green lines drawn from the attractor point along the normal of the circles, which corresponds to the orientation of the circles.
