noun. Ligature of the lower‐case letters c and t. A round, leftward line from the up‐most tip of t is drawn onto the upper end of c, joining the letters.

noun. carat

"The analogy with (4), already close, can be made even closer by introducing τ = √ -1 ct, τ′ = √ -1 ct′, for now s2 = x′2 + y′2 + z′2 + τ′2"