Abnormal pressure on a nerve from a hard structure such as a bone or disc. This can lead to nerve damage if it continues long enough. Nerve Entrapment: Abnormal pressure on a nerve from a tight muscle. This doesn't lead to nerve damage.
"Pinching" a nerve by putting too much pressure on it. For example, the sciatic nerve may be painfully compressed by a ruptured disc in the lower spine causing sciatica. See the entire definition of Nerve compression