Kanchan (嵌張) is a tenpai shape that waits on the one tile filling the gap in the middle of an incomplete run. You hold the two outer tiles of a sequence (for example 4m and 6m) and need exactly the central tile (5m). Because only a single value completes it, the wait accepts at most four tiles, as narrow as a penchan.
Any two same-suit numbered tiles two ranks apart form a kanchan (2-4 waits on 3, 5-7 waits on 6, and so on). On a winning hand this closed middle wait is worth 2 fu, the same bonus as a penchan or a tanki wait, unlike a ryanmen which scores 0 fu. For that reason a kanchan wait disqualifies pinfu, which requires a ryanmen wait.
A kanchan has narrower maximum ukeire than a ryanmen. Improving it can increase speed, but the correct choice also depends on shanten, yaku, dora, visible copies and discard danger. On defense, suji does not protect against kanchan waits, so a suji tile is never guaranteed safe for that reason alone.