1. What does it mean?
천장 literally means ceiling. In gacha discussion it refers to the point at which a system guarantees a specified rarity, item, or exchange opportunity after enough draws, rather than leaving every attempt entirely to chance.
Also written or said as cheonjang
2. The nuance
The guarantee may not be the exact character you want. Thresholds, carryover, and resets differ by game and banner; the term alone does not define those rules.
3. Examples
“천장이 이월되는지는 게임 안내에서 확인해야 해.”
You need to check the game's guide to see whether the pity count carries over.
“이번에는 운 좋게 천장 전에 나왔어.”
This time I was lucky and got it before reaching the guarantee threshold.
Sources
References support the meaning and usage. Example sentences are written for NoCapWiki.