A bass line is a series of notes. Bass tones are low ranging pitch/frequency. Various natural instraments can create bass range notes such as drums, guitar, and cello. A clasic bassline is the bass guitar that accompanies the rock band classic instrament list.

Electroaccoustic modeling enables countless bass sample instruments. In electronic music a classic bassline may be a 303 bassline. In any given ammount of time frequency has pulsations bass is a very pulsy sound that can seem to be plodding rather then sharp/snappy, and so the baseline tends to sound like it is plodding. You can say any bass tones in a sequence (melody) that is in the bass range is a bassline. Therefor drums could be a bassline although they tend to be classed as percussion. Alternatively if you take an instrament that classically plays in the treble clef and take that to the bass clef it could then technically be considered a bassline. So to recap a bassline is a series of notes in the bass range and/or a melody played by a classic instrament which is considered to make basslines.