The resolution is defined as points/pixels per length unit, usually inch.
In image quality, it is measured in ppi - pixels-per-inch or dpi - dots-per-inch. That image has resolution of 150 dpi.
In printing, some printers have different resolutions in both directions. In that case it is used X x Y dpi notation sayng it has resolution of X dpi in one direction and Y dpi in the other.
The value of 15 MPx says that the chip has 15 milons of pixels, that means that for usual 3:2 ratio it is capable of taking pictures with 5000 x 3000 pixels. When making 5" wide print, the resolution will be 1000 dpi.
Your good point is that one X MPx resolution can be obtained using many different frame sizes. It is same nonsense as measuring the screen size only by its diagonal length! But, luckily for photography, the usual ratio is 3:2 so the values are directly comparabe (if you ignore cameras developped from mid-format with frame size of 6x6 cm). The screens are not comparable at all.
On the linguistic side of the question, megapixel is a unit. The same unit as mile is.
- The image has resolution of 15 MPx. / The road has length of 15 miles. (it is 15 miles long.)
- This is 15MPx image. / This road is an 15mile road.