A cluster of terms describes relationships as spiritually pre-arranged, and they are routinely run together though they carry different claims and different histories. The most damaging of them, the twin flame, is treated separately in these notes and is explicitly not the same as a soulmate; the rest are worth distinguishing.
A soulmate is the oldest and mildest. The romantic modern sense, one person uniquely suited to another, is often traced to Plato's Symposium, where Aristophanes tells a myth of beings split in two who seek their other half, but that is a source borrowed after the fact; the English word in its current meaning is a nineteenth-century coinage (Coleridge used it in 1822), and it spread through Spiritualist and New Thought writing on affinity between souls. In common use it need not even be romantic and carries none of the compulsion the twin-flame idea does.
A karmic partner, or karmic relationship, applies an Indian-derived vocabulary of karma to a specific relationship: the two are said to have unfinished business from a past life, and the relationship, often a difficult or short one, exists to work it out. The traditional doctrines of rebirth do not, in fact, offer this kind of named, recallable person-to-person accounting; it is a modern Western adaptation of karma to the concerns of the self-help relationship.
A soul contract generalises the idea into a pre-birth agreement: the soul is said to have planned, before incarnating, the major people and difficulties of its life, so that even painful relationships and losses were chosen in advance for their lessons. The vocabulary was popularised by books such as Caroline Myss's Sacred Contracts (2001) and Robert Schwartz's Your Soul's Plan (2007).
The three share a structure and a risk. Each reframes an ordinary relationship, including a harmful one, as fated and purposeful, and the more strongly the framing is held the more it can work to keep a person inside a situation they might otherwise leave, by recasting distress as a lesson that has not yet been learned. As spiritual consolation the vocabulary is old and human; as counsel for a specific bad relationship it can do the same work the twin-flame idea does, if more gently.