What is 'k' in sequencing?

When a DNA sequence is sequenced, I've only ever dealt with A,T,C,G and N which indicates un-identifiable bases. However, I came across a 'k' recently and I had asked another researcher who gave me an answer for what 'k' represents but I don't quite recall. It can't just be an anomaly in that one sequence file. Any ideas?

• K is also the lysine amino acid (and k-mers are something else of course) Dec 29 '19 at 21:28

So, as you can see above, K means "Either G or T".