Whenever I have seen the word “lamp” used in the context of lighting, I have often seen the word used in 2 different contexts. In one definition, I have seen the word “lamp” being used to refer to an entire light fixture such as in a “hanging lamp” or a “table lamp”. In another definition, I have seen the word “lamp” being used to define only the light source itself as in a “mercury vapor lamp” or a “fluorescent lamp”. Why is it that the word “lamp” in the context of lighting refers to an individual light emitting source or an entire light fixture containing the light emitting source?

In some cases, I sometimes feel like I end up confusing people whenever I say something like “This table lamp is using a fluorescent lamp”.

I generally use the word "lamp" to refer to a light source or "bulb" (which I hate by the way). When referring to a light fixture, I generally use the word "Luminaire" (generally for streetlights) or fixture. 

In the United States, the word “lamp” is most often used to describe a portable, plug-in light fixture, usually one that is placed on a table or the floor and is rather tall and/or has a shade around the light source. Most people call the light source the “bulb” outside of professional circles, even if the bulb is not bulbous in shape, like a fluorescent bulb. I call fluorescents “lamps” because it doesn’t really make sense to call them bulbs, and I call HID and halogen things “lamps” out of respect, but I usually still call incandescents “bulbs” colloquially because that’s the classic term used for them. I do use the word “lamp” to describe a tall plug-in tabletop or floor fixture with a shade, but I use the word “fixture” to describe any other light bulb housing. I also use the word “lamp” to describe a light fixture that uses fire to produce light, such as an oil lamp, regardless of whether it’s portable or not.

I was told by the street lighting engineer who taught me almost everything, Steve, that a ‘bulb’ was the lamp and the fitting that I always called the ‘head’, was obviously the lantern, being an old boy electrician he was very stern about this 😁
But to normal people, they put a new bulb in the lamp!🤣, which I must admit sounds more proper to me, but in the street lighting world that Steve worked in, (he’s now retired), it was normal for them to fit a new lamp in the lantern! 🥳

The world "Lamp" in English, used to describe either a lighting fixture or a light bulb.

Pretty much everywhere outside of this website, "Lamp" has been used to mean a floor/table lamp. All other light fixtures are called, well, light fixtures . The only places I had seen the word "lamp" used to refer to the light source was on projectors and fluorescent lamp packaging. I thought it was the word the British used for "light bulb" at the time.

But pretty much everyone calls A19 and halogen lamps "light bulbs", whether they're bulbous or not, and fluorescent lamps are "fluorescent bulbs". I try to use those terms when I'm talking to other people about lights.

Using lamp to refer to to the actual light emitting device (light bulb) instead of the entire assembly seems to be a more technical thing and is used more on technical lighting literature, electrical and lighting plans for a building, and certification/electric code references.

Usually if lamp is used that way, the assembly that contains the lamp is usually referred to as the luminaire or the fixture in some more informal cases.

In most streetlighting plans here, the fixtures are typically referred to as luminaires but it seems like different places have slightly different terms.

Over here the contractors refer to $)"columns" and "luminaries".

In Russian-speaking countries, the world 'lamp' is too used with a meaning of 'light-source element' like a fluorescent tube or HID lamp, or in the meaning of a light fixture. Or the use is even extended to electron tubes and devices employing them like radios or guitar amps, like 'ламповый усилитель'.

At the home, a term 'lamp' is used in relation to the light fixtures but too, this mainly applied to portable and tabletop lamps. We usually call wall and hanging fixtures more specifically, like 'бра' and 'люстра'. Outdoor streetlight in a plain language is 'фонарь', somewhat old-school sounding. Semi-official name for any luminaire is 'светильник', but it is relatively rarely used in a plain language, more like a term that you encounter in a price-list or some official documents, or related to office lighting.