- Dielectric grease is a silicone-based gelatinous lubricant.
- It looks very similar to petroleum jelly, with many similar physical properties.
- It will not dissolve in water and is used as a sealant and corrosion inhibitor.
- It can degrade silicone rubber over time, however, so be careful using it on rubber components.
- It is also non-conductive, and will insulate electrical connections.
In cars, it's most commonly used on the rubber sealing boots of spark plugs. It's also quite useful on trailer plug connections, and on any other external wiring connections as well: keeping water out of these areas will prevent short circuits and simplify vehicle maintenance. It can also be used on outdoor connections or damp area connections on circuits on your home and outbuildings.
Dielectric grease is also excellent for sealing threaded battery compartments on devices like flashlights. A thin smear on the threads will keep water, condensation, and debris out, preventing damage to the battery compartment and prolonging the life of your tools.
One of the best things about dielectric grease is the price. $5 gets a 3 oz tube, which is a huge supply for the average user.
More information about dielectric grease can be found here, here, and here.
I always love when discussion allows me to learn new things, especially about something this underrated and useful.