What is Isotype?

Isotype definition and meaning on Dictionary terms:
a drawing, diagram, or other symbol that represents a specific quantity of or other fact about the thing depicted: Every isotype of a house on that chart represents a thousand new houses.
a statistical graph, chart, diagram, etc., that employs such symbols.
Biology. any of two or more separate populations of the same or a similar type.
Immunology. any antigenic determinant that is common to all individuals in a species.