drip line

Short definition from Wiktionary

Noun

1. 1
   On the table of nuclides, the limit beyond which an atomic nucleus will immediately decay by emitting a proton or neutron.
2. 2
   The area defined by the outermost circumference of a tree canopy from where water drips onto the ground.
3. 3
   A pipe that has small holes at intervals, used to irrigate crops.