Time portal

A time portal is a gateway between two or more points in time and space. Such portals could be either artificially-created devices or fields or naturally-occurring rifts in the space-time continuum. 

Perhaps the most important time portal discovered by the United Federation of Planets was the Guardian of Forever. Other portals included one on the planet Golana, the atavachron device on Sarpeidon and the Well of Tomorrows on Algol II. (,, )

The Orb of Time also functioned as a form of time portal.