Readability vs Legability

What's the difference between READABILITY and LEGIBILITY?  What do they mean to you as a designer? 
     
       Readability = Able to read
     Legibility = Able to see 
       As a designer this is important. We want our designs to be both readable and legible.  Lets say I created a billboard, because its a billboard and huge its going to be legible. You can see it, you know its there. But I used a 12 pt size font. So you can see the billboard is there, but your not going to be able to see it. It is not readable. No one is going to know what that billboard says. So as the designer, it is important to have both factors. You want the audience to see the advertisement is there and be able to read it. no matter how great the design is if its not readable it is useless.