pioneer tunnel

noun

variants or pioneer bore

: a small tunnel parallel to a main tunnel and well in advance of the completed main tunnel so that by crosscuts to the line of the main tunnel several headings can be exposed and work expedited

The Ultimate Dictionary Awaits

Expand your vocabulary and dive deeper into language with Merriam-Webster Unabridged.

1. Expanded definitions
2. Detailed etymologies
3. Advanced search tools
4. All ad-free

Discover what makes Merriam-Webster Unabridged the essential choice for true word lovers.

Start Your Free Trial Now