Tsarevich was a title given to the sons of tsars. The female equivalent was tsarevna.
Under the 1797 Pauline house laws, the title was discontinued and replaced with tsesarevich for the heir apparent alone. Wikipedia
Past holders: Alexei Nikolaevich, Tsarevich of Russia, Alexei Petrovich, Tsarevich of Russia, Dmitry of Uglich, and more