syn- /sin-/, sy- /si-/, syl- /sil-/, sym- /sim-/ or sys- /sis-/

Signifying together, with

• boiler rooms
• boilers
• boiler suit
• boiler suits
• boilery
• boiling
• boiling point
• boiling points
• boilings
• boiling-water reactor
• boiling-water reactors
• boil off
• boil over
• boilover
• boilovers
• boils
• boing
• boinged
• boinging
• boings
• boink
• boinked
• boinking
• boinks
• boisterous