Involucrum

noun

1. An enveloping sheath or envelope, such as the sheath of new bone that forms around a sequestrum.
2. In zoology, a kind of sheath or involucre about the bases of the thread-cells of acalephs.
3. In botany: Same as involucre.
4. Same as velum.
5. In pathology, a wall of new osseous tissue inclosing the sequestrum in a case of necrosis.