A Mermaid silhouette is a dress that closely hugs the body from the bodice to the knee (or just below the knee), where the skirt then flares away from the body to the hem.
A Trumpet gown is tight-fitting through the bodice, tapers close to the legs to the mid-thigh (or just above the knee), then gradually flares to the hem. Trumpet gowns are generally less constrictive, allowing more freedom of movement than the Mermaid styles.
Mermaid and Trumpet styles are very similar to Fit and Flare. The differences are Trumpet flares gradually at the mid-thigh or just above the knee; Mermaid flares at or below the knee; Fit and Flare flares right below the hip to mid-thigh.