Correction fluid is an opaque, white fluid applied to paper to mask mistakes. Once dried, it can be written over. It is typically packaged in small bottles and the lid comes with an attached brush that dips into the bottle. The brush or pen is used to apply the fluid onto the paper. As the fluid is highly volatile (i.e. it dries quickly), the unused contents of the bottle often dry out and become too thick to use. Manufacturers of correction fluid will often sell corresponding thinners to dilute the bottle.
More recently it has been made available in pen form. In the pen form, the pen is spring loaded and is dabbed onto the paper. The pen form has the advantage that it applies the fluid more evenly and thinly, and does not dry out in the bottle so quickly.
Correction fluid is commonly referred to by the leading brand names. These brands include: