Nucleophilic substitution reaction where the leaving group leaves before the nucleophile attaks. This reaction is also called a dissociative reaction (a term that is more typically used in complex chemistry substitution reactions).
The graph was obtained from : https://chem.libretexts.org/Courses/University_of_Illinois_Springfield/UIS%3A_CHE_267_-_Organic_Chemistry_I_(Morsch)/Chapters/Chapter_07%3A_Alkyl_Halides_and_Nucleophilic_Substitution/7.12%3A_The_SN1_Mechanism