An analogue in any dimension of the triangle or tetrahedron: the convex hull of n+1 points in n-dimensional space.
"In higher-dimensional geometry, a simplex is defined as the convex hull formed by connecting n plus one points in an n-dimensional space, generalizing familiar shapes like triangles and tetrahedra to any dimension."
allowing communication in only one direction at a time, or in telegraphy allowing only one message over a line at a time
"simplex system"
having only one part or element
"a simplex word has no affixes and is not part of a compound--like `boy' compared with `boyish' or `house' compared with `houseboat'"
Single, simple; not complex.
"The doctor noted that the patient's heart was in simplex rhythm after the surgery resolved the previous complications."