A metaball, sometimes called a "blobby object", is a modeling object that employs "implicit representations" as an alternative to parametric calculations of a surface normal. Implicit representations define the surface of an object as the zero contour of a function containing 2 or 3 variables.
This method is advantageous for operations like blending and metamorphosis. The cool thing about metaballs is their expediency for being combined.
Visualize, if you will, two globs of mercury floating toward each other in weightlessness. As one surface comes in contact with the other, they slowly meld together.
A metaball has a surface and a "sphere of influence" surrounding it. When two metaballs near and the spheres of influence intersect, the effect of each point on each surface on one another is calculated, and the surface is deformed accordingly (deformation is determined by distance and angle, in case you were wondering). As they draw nearer, the surfaces touch and form a "bridge", akin to drops of liquid.
Another neat way to use metaballs is to give them a "negative weight", where they do the same thing but inverted. They become invisible fields that create depressions in other metaball surfaces.
Metaball is also considered a careless misspelling of the word "meatball" by laymen.