Which hair styling product is often used to define curls?

Mousse is a popular choice for defining curls because it is specifically formulated to enhance volume and provide a lightweight hold. The foam texture allows for even distribution through the hair, which helps to enhance and support the natural curl pattern without weighing the hair down. Mousse typically offers flexibility, allowing curls to maintain their bounce and movement, which is essential for curly hair.

While gel and spray gel can also define curls, they tend to provide a firmer hold, which might result in a crunchier texture that some people may want to avoid. Hair serum is primarily used for smoothing and adding shine rather than defining curls. Thus, in the context of curl definition, mousse stands out as the most effective option due to its unique formulation and benefits for curly textures.