For Sanctuary fans, it refers to a ship (not canon YET) between Helen Magnus and Nikola Tesla.
Over the two seasons and a half of the TV series, there have been a lot of clues that the relationship between Nikola and Helen is more than friendship. For example, they have both shown concern when the other was in danger, not to mention the fact that the UST between them is more than obvious:
Nikola: (to Helen, in "Sleepers") : "I've often wondered what this moment would be like. Me. You, tied up. It's a shame you brought the children."
Helen: (in episode For King and Country): "I like the whipping Tesla idea." (btw, she had just found out that she was dying when she said that)
Not to mention that he confessed his love for her, in "The Five".
The two have shared a kiss (sort of) in "The Five" and have hugged eachother in "Animus" (both of this situations were caused by Nikola's tricks, but neither of them seemed to bother Helen too much).
Helen and Nikola have known each other for approximatively 123 years and over the years Nikola had saved her life two times, both times risking his life to protect her. In "Animus", Helen also refused to leave Nikola alone in a situation that would have probably killed him.
The only two reasons they are not together yet is because she is too stubborn to admit her feelings and that Nikola really has a problem with timing sometimes...but this doesn't change the fact that they are perfect for each other.
Teslen is my favourite Sanctuary pairing. I ship them so badly!