Okay, lets try a third definition.
Clear dewaxed shellac (1 pound cut when I do it) works as a sealer, but is only one of the choices
Lacquer with stearates works as a sealer, but I haven't used that since I went to water-based finishes.
Basically it is any transparent material that fills some of the wood pores (i.e. sealer), providing a very smooth surface with good adhesion for building the following coats of finish. To be sure it is flat, it generally is a soft finish that is easy to sand very smooth. My water based lacquer can be thinned a few percent and is a good sealer, but it doesn't "pop" the color of walnut and some other woods as well as other sealers like shellac. My favorite is a custom sealer product from my finish vendor - covers well, sands easily, and brings out the color as well as any other sealer I have used - but it is neither shellac nor lacquer.