NSU (non specific urethritis) occurs generally in men and is simply an inflammation of the urethra. Unlike other STIs, it's defined by its symptoms - the inflamed urethra - rather than the presence of a specific bacteria or virus that causes it. So if you have an inflamed urethra and the doctor can't specify what's causing it, you have NSU.
Aside from an inflamed urethra, a man with NSU might get a white or clear penile discharge sometimes accompanied by feeling of burning when urinating.
NSU could be caused by a number of things. A common cause of NSU is Chlamydia, so unprotected vaginal, oral or anal sex could cause it. Energetic sex of any kind or even enthusiastic masturbation might also inflame the urethra, as might something as simple as riding a bicycle for long periods at a time or not drinking enough fluids. Excessive alcohol and rarer but treatable bacteria have also been implicated as a cause.
To test for NSU a doctor will normally take a swab from the urethra and look at the swab under a microscope. If the doctor thinks the inflammation might be caused by Chlamydia then you'll probably be prescribed antibiotics. Even if your test comes back negative for Chlamydia the antibiotics are worth taking to treat the rarer
If a partner has been diagnosed with NSU you can go to your local NHS clinic.