Q: Audio input on mac (MacBook Pro (Retina, 13-inch, Mid 2014) that has only ONE audio port, for headphones, and NO INPUT PORT.

Connect an audio interface to a USB port to have an audio input, or connect an input splitter to the headphone port. You need additional hardware to have an audio input.

What are you trying to attach?  What kind of connector?  Last week I wanted a USB converter for an analog microphone. I purchased a Sabrent converter from Amazon and it seems to work okay for Dictation purposes.  I think it will only work with TRS type plugs which is the most common style.  You may wish to look up the differences between TS, TRS, and TRRS jacks first if you have not already.

Once you have it attached you need to go to sound preferences and configure USB as audio-in.

For a guitar or another instrument with a wide range of frequencies I'd use an audio interface. For my guitar Ihave an Apogee Jam, and the sound is great.

I first used the iMic adaptor, but the latency was too bad.

Audio interfaces are much more expensive than input splitters but the audio quality is better.

From Apple's specs on that computer there is no audio input other than the two built-in microphones. There is only audio output (headphones).

To me a "splitter" is something where I can take, say, two headphones and plug them into a single jack. It is two jack sockets leading to one jack. Since your jack is audio-out only this won't work, and even if it could a single socket cannot be used for two purposes simultaneously.  You may not even need a splitter of any kind but what you definitely need is a converter.

The device to which I was referring lets you take either an analog headphone or a microphone and convert the signal into a digital form that interfaces with the computer through the USB port.  Yes, the cheaper ones are limited in quality but you never got around to telling us exactly what you needed this for.

The headphone port of the MacBook Retina 13", Mid 2014 has support for the Apple iPhone headset, see: https://support.apple.com/kb/sp703?locale=en_US

• Headphone port
• Support for Apple iPhone headset with remote and microphone
• Support for audio line out (digital/analog)

And that means, it can be used to connect an input splitter like this one:

2x 3-Pin 3.5mm Headset Splitter Adapter - M/F

http://www.amazon.com/dp/B004SP0WAQ).

But the audio quality of a true audio interface will be much better.

It may be like the Superdrive where input requires an Apple product such as the specifically mentioned i-Phone microphone/headset. This topic How can I record guitar directly into my macbook pro "13 retina display laptop inexpensively? (in which you posted) suggests that normal input functionality was discontinued post-2012 (Frank Caggiano's posts).

Here is an article about the older models I believe. The article does not mention which models but it is archived so presumably at some stage this feature was discontinued.  MacBook and MacBook Pro (13-inch): How to use the combination audio port for an audio input device - Apple Support  Even then it is important to note different levels of input signal.  It is possible this will not work for a microphone since it specifies line level input.  I tried using the audio-in socket on my late 2008 MacBook with a microphone and the signal strength was inadequate.

Yes, the image you provide is an analog splitter.  Even if the OP can use a microphone the splitter won't be all that helpful other than allowing both the microphone and headphones to be plugged in at the same time but still requiring sound settings to be changed in system preferences when changing from one function to another. Since this is a later model MacBook pro the OP will still need a USB digitizer, particularly if the input is a microphone.

Yes, I remember the old discussion with   Frank Caggiano.  The older Macs had a combination input/output port, that was more versatile than the current headphone port. We could switch between input or output using the system preferences.

The current headphone port jack requires a plug with the three contact rings to connect a microphone as well as a headset, as shown in the screenshot.

Yes, that is the TRRS style jack I mentioned in an earlier post.