What is an E14 European Intermediate bulb?

There are four commonly used thread size groups for mains supply lamps:

1. Candelabra: E12 (12mm) North America, E11 (11mm) in Europe
2. Intermediate: E17 (17mm)  North America, E14 (14mm) in Europe
3. Medium or standard: E26 (26mm) in North America, E27 (27mm) in Europe
4. Mogul: E39 (39mm) North America, E40 (40mm) in Europe.

The E14 is an intermediate base used mostly in Europe and Chiana, with a diameter of 14 mm.

The Intermediate base bulbs - E14 and E17 - are especially used in small table lamps and novelty lighting, and occasionally the lights on newer ceiling fans. 

Although E14 and E17 are both Intermediate bases, the bulbs are NOT interchangeable due to the difference in base size.  Also, the E14 will not fit in the smaller candelabra socket (E12 - 12mm).

Q&A: Can an E14 bulb base work in an E27 mount?
You can use an E14 bulb in an E27 mount only with a base adaptor. In that case is very important to verify the fixture and the bulb has the same voltage range.