An acnode is an isolated point in the solution set of a polynomial equation in two real variables. Equivalent terms are " isolated point or hermit point".
For example the equation
:$f(x,y)=y^2+x^2-x^3=0$
has an acnode at the origin, because it is equivalent to
:$y^2\; =\; x^2\; (x-1)$
and $x^2(x-1)$ is non-negative only when $x$ ≥ 1 or $x\; =\; 0$. Thus, over the ''real'' numbers the equation has no solutions for $x\; <\; 1$ except for (0, 0).
In contrast, over the complex numbers the origin is not isolated since square roots of negative real numbers exist. In fact, the complex solution set of a polynomial equation in two complex variables can never have an isolated point.
An acnode is a critical point, or singularity, of the defining polynomial function, in the sense that both partial derivatives $\backslash partial\; f\backslash over\; \backslash partial\; x$ and $\backslash partial\; f\backslash over\; \backslash partial\; y$ vanish. Further the

Hessian matrix
In mathematics, the Hessian matrix or Hessian is a square matrix of second-order partial derivatives of a scalar-valued Function (mathematics), function, or scalar field. It describes the local curvature of a function of many variables. The Hessi ...

of second derivatives will be positive definite In mathematics
Mathematics is an area of knowledge that includes the topics of numbers, formulas and related structures, shapes and the spaces in which they are contained, and quantities and their changes. These topics are represented in mode ...

or negative definite In mathematics, negative definiteness is a property of any object to which a bilinear form may be naturally associated, which is negative-definite bilinear form, negative-definite. See, in particular:
* Negative-definite bilinear form
* Negative-de ...

, since the function must have a local minimum or a local maximum at the singularity.
See also

*Singular point of a curve
In geometry, a singular point on a curve is one where the curve is not given by a smooth function, smooth embedding of a parameter. The precise definition of a singular point depends on the type of curve being studied.
Algebraic curves in the plan ...

*Crunode
In mathematics, a crunode (archaic) or node is a point where a curve intersects itself so that both branches of the curve have distinct tangent lines at the point of intersection. A crunode is also known as an ''ordinary double point''.
For a ...

* Cusp
* Tacnode
References

* Curves Algebraic curves Singularity theory {{algebraic-geometry-stub es:Punto singular de una curva#Acnodos