Sunblock (also commonly known as sun screen, sun lotion, sun cream or block out) is a lotion, spray, gel or other topical product that absorbs or reflects some of the sun's ultraviolet (UV) radiation on the skin exposed to sunlight and thus helps protect against sunburn. Skin lightening products have sunscreen to protect lightened skin because light skin is more susceptible to sun damage than darker skin.

Sunscreens contain one or more of the following ingredients:

Organic chemical compounds that absorb ultraviolet light.

Inorganic particulates that reflect, scatter, and absorb UV light (such as titanium dioxide, zinc oxide, or a combination of both).

Organic particulates that mostly absorb light like organic chemical compounds, but contain multiple chromophores, may reflect and scatter a fraction of light like inorganic particulates, and behave differently in formulations than organic chemical compounds. An example is Tinosorb M. Since the UV-attenuating efficacy depends strongly on particle size, the material is micronised to particle sizes below 200 nm. The mode of action of this photostable filter system is governed to about 90% by absorption and 10% by scattering of UV light.

Depending on the mode of action sunscreens can be classified into physical sunscreens (i.e. those which reflect the sunlight) or chemical sunscreens (i.e. those which absorb the UV light).

Medical organizations such as the American Cancer Society recommend the use of sunscreen because it prevents the squamous cell carcinoma and the basal cell carcinoma. However, the use of sunscreens is controversial for various reasons. Many sunscreens do not block UVA radiation, which does not cause sunburn but can increase the rate of melanoma, another kind of skin cancer, so people using sunscreens may be exposed to a high level UVA without realizing it.

It is often colloquially called sun tan lotion due to the product's similar name, although sun tan lotion is used to absorb UV rays rather than block them.