I am looking for a high density connector that can carry a 150Vrms AC signal at 10khz. I found this connector.

https://www.samtec.com/products/ercd

It says that it has a 50 ohm impedance (I assume they mean on the input and output). For my frequency, is this impedance ignorable? Or, would this still mean that I will not get all the voltage at the output?

 

That refers to the characteristic impedance, which is important at higher frequencies where the connector characteristic impedance should match that of the connecting wire.
At 10kHz it will have negligible effect.

 

At high frequencies, it cannot be ignored. Everything with the ability to transmit an AC signal has a 'characteristic' impedance. You are unlikely to be able to measure this impedance with a multimeter. Now when an AC signal encounters an impedance discontinuity, where the characteristic impedance changes abruptly from one value to another value, part of the signal is transmitted through the discontinuity and part of the signal is reflected back to the source. What the connector specification is telling you is that you can minimize the disruption to the signal by connecting a 50 Ω source to the input side and a 50 Ω load to the output side. Any other combinations may create disruptions in the signal which must be evaluated.

If 10 kHz is the highest frequency component, as it would be with a sinewave, then you ar probably OK. A 10 kHz squarewave on the other hand WILL have higher frequency components, and that could be a problem.