Gradient is a measure of the degree of slope. While gradients on naturally occurring features (hills!) can be hard to avoid, they can (and should) be mitigated, particularly by extending routes. An example is shown below.

Low-gradient cycle ramp, Utrecht 

It is important that gradient is kept to a minimum on cycle routes which deal with designed features (for instance, passing over or under roads). Below is a table of 'maximum lengths' for gradients of increasing steepness, from Cycle Traffic and the Strategic Road Network.

Maximum gradients IAN 195/16