If something is inevitable, it will definitely happen, like death or tax season.
Inevitable comes from the Latin word inevitabilis, which means unavoidable. If you say something is inevitable, you give the sense that no matter what scheme you come with to get around it, it's going to happen sooner or later. You can use all the skin products you want, but wrinkles are inevitable.
adj incapable of being avoided or prevented
controlled or decreed by fate; predetermined
ineluctable, inescapable, unavoidable
impossible to avoid or evade:"inescapable conclusion"
unavoidably determined by prior circumstances