tet·rode 
(tĕtrōd′)
(tĕtrōd′)n.
A four-element electron tube containing an anode, cathode, control grid, and additional electrode.
- on sale
- onscreen
- on-screen
- onset
- onshore
- onside
- onside kick
- on sight
- onsite
- onslaught
- on soapbox
- on (someone's) account
- on someone's account
- on (someone's) back
- on someone's back
- on (someone's) case
- on someone's case
- on speaking terms
- on spec
- onstage
- on standby
- on steroids
- onstream
- on-stream
- on stream