There were many many different species called "viola" in the Middle Ages. In Germany alone, I have found about a dozen plants (different species) with violet-colored flowers that were labeled "viola". Before Linnaeus it was a more general term (usually describing the flower color) than it is now.
Some of the common names for Viola tricolor include Trinitatis, Heart's Ease, Styvmorsviol, Phlozion (and numerous more poetic ones like "love in idleness").
There were also plants shaped like Viola (with a variety of flower colors) that were called viola. For example, a different species, Caltha palustris, which is usually called "marsh marigold" was also sometimes called "viola martia".
Some of the common names for Viola tricolor include Trinitatis, Heart's Ease, Styvmorsviol, Phlozion (and numerous more poetic ones like "love in idleness").
There were also plants shaped like Viola (with a variety of flower colors) that were called viola. For example, a different species, Caltha palustris, which is usually called "marsh marigold" was also sometimes called "viola martia".