By the time most people say “I think we should break up,” the relationship has usually already been over for a while — emotionally, if not officially. The slow fade is the gap between those two moments: the period where affection quietly downgrades to habit, and habit quietly downgrades to obligation.
It shows up in small substitutions you barely notice while they're happening. You stop sharing the small, unimportant parts of your day because it doesn't feel worth the effort anymore. Physical affection becomes something that happens on a schedule instead of an impulse. Conflict starts getting avoided instead of resolved, not because things have gotten more peaceful, but because neither person believes the conversation will change anything.
The slow fade is dangerous precisely because nothing about it looks like a crisis. There's no single bad night to point to, no clear villain, which makes it easy to keep going for far longer than the relationship actually has anything left in it. Both people can spend a year technically together and functionally alone.
If you recognize this pattern in your own relationship, the useful question isn't “when did this start?” — it's rarely answerable and doesn't matter much. The useful question is whether you're willing to have the honest conversation now, or whether you'd rather let the fade keep fading until it makes the decision for you.