Definition
TRAJECTORY OPTIONS SET (TOS)
A TOS is an electronic message, submitted by the operator, that is used by the Collaborative Trajectory Options Program (CTOP) to manage the airspace captured in the traffic management program. The TOS will allow the operator to express the route and delay trade-off options that they are willing to accept.
Source: FAA Pilot/Controller Glossary (verbatim from the Federal Aviation Regulations).