The biggest of the standard tuned ukuleles. As well as being longer than the other strings, they are also thicker.
Tenor ukulele strings are often low-G, so make sure you check whether the strings are low-G or re-entrant before you buy them - I've ended up with strings I didn't want before.
The low-G string will often be wound (nylon strings with steel wrapped round them to make them thicker). In some cases, the C string on tenor ukuleles will also be wound. Personally, I don't like these as they create a different tone from the pure nylon strings.
High G Tenor Strings On eBay