RMS Richard M. Stallman
RMS Root-Mean-Square (power measurement)
RMS Rack Mount Space
RMS Radar Maintenance Spares
RMS Radiation Monitoring System
RMS Radio Marketing Service (Hamburg, Germany)
RMS Rail Maintenance Service (KPR locomotives)
RMS Railway Mail Service
RMS Rainwater Management Solutions
RMS Rake Management System
RMS Ramp Metering Station (traffic management)
RMS Range Management Squadron
RMS Range Measuring System
RMS Rapid Mobile Surgical (hospital)
RMS Rapid Multilingual Support (DARPA Babylon program)
RMS Rate Monotonic Scheduling
RMS Raw Material Supply
RMS Raw Materials Surcharge (steel)
RMS Raytheon Missile Systems
RMS Ready Maintenance Stores
RMS Receivable Management Services
RMS Recipe Management System (manufacturing)
RMS Reconfigurable Manufacturing System
RMS Reconnaissance Management System
RMS Record Management Services
RMS Records Management Society
RMS Records Management System
RMS Recovery Management S ...more

RMS Root Mean Square.

Railway Mail service.

RMS stands for "Root Mean Square", which is fancy mathematical formula for finding the average amount of power an amplifier can continuously produce