The color of the flower
This shade is meant to match the flowers of the lavender plant, so it is often called floral lavender. It is much stronger than the web color lavender, #E6E6FA, which looks almost white. The Wikipedia article on lavender dates the first use of the word for a color in English to 1705.
The 1930 sample
#B57EDC matches the lavender swatch in Maerz and Paul's A Dictionary of Color, printed in 1930. Before computers, that book was the usual reference for color names. Wikipedia calls the shade a medium violet. Mixing violet paint and white paint half and half would give about this color.
Close relatives
Lavender purple is grayer and a bit darker. Wisteria is paler and softer. Black text on floral lavender has a contrast ratio of 6.98 to 1, so it is easy to read.

