IMPROVISATION

 Improvisation is the instantaneous creation of something based on the skills and knowledge available to the individual. 
Improvisation can be a bassline, a melody, solo, even just one note. In musical terms it really is based on what an individual knows, what they can hear and what they can execute instantly. The repeating of learnt lines and licks, while often included in an improvised piece of music, is not Improvisation.
When you hear a great improviser, regardless of the type of music, or whether it's a bassline, solo or chord pattern, that individual will be drawing on their knowledge and skill in all areas of music. The information will have been learnt to the level that it is not an intellectual process, but more of a natural response to the situation.
As Bass Players, we are often blind to the fact that we are improvising in many situations. Our ability is only stunted by our level of knowledge and our insecurity. The feeling that now we are exposed and must do something breathtakingly complicated.

 A bassline or a solo, can both be improvised.  You could change the feel, dynamics or notes and you are improvising. 
One of the biggest problems for Bass Players when it comes to playing an improvised solo is that they totally forget that they have already been improvising.  Now they are just expanding on the process. We hear a guitarist or saxophonist solo, and when its our turn, we feel under pressure to play with the same speed.
A true solo is not about impressing people with your latest slap technique. That is not a solo but a pre worked line. And there is nothing wrong with that, but that is covered within  "Technique Studies".
If you want play a truly improvised solo, then the ego must go out of the window.  You must want to make statement.
The study of Melodies is vital to creating meaningful and honest solos. After all, what we are trying to do if not play a solo as good as the melody, or better!