Ah, good question! Both are used almost interchangeably. ISEA, the International Safety Equipment Association, has set forth a recommendation on the topic. Lanyards are to be used in Fall Protection for People. Tethers are for tools used to Prevent Dropped Objects.
So, to keep it simple:
- Tether your Tools
- Lanyards are Lifelines
For more information, check out the new ISEA 121 standard just published for public review and comment: