What is the definition of guest lecturer? Is it worth putting on CV? I'm a couple years out of undergrad, have my Master's and work at a research org. A former professor from undergrad invited me to come teach a class with him next month--I told him I'd be in town for a day, just thinking we'd get coffee, but he invited me to come teach and talk about some of the relevant research I'm currently doing.  

Is this "legit" enough to mark down on my CV as being a guest lecturer? If so, is it actually worth putting on my CV, will it hold any weight whatsoever when I apply to PhD programs in a couple years?
The short answers are: (1) yes, this fulfills my definition of a "guest lecture" (that being you were invited by the module or subject leader and that you delivered a lecture) and (2) yes, you should include this in your CV.

Let me explain. I've chaired departments and academic units before and have been involved in hiring decisions. Academic jobs outside of a research fellowship often require responsibilities in three broad areas: (1) research, (2) teaching and (3) administration/service. It is often the case that quite a lot of emphasis is placed on research skills and with good reason. However, teaching is also a skill. Having been a guest lecturer signifies to me that you've garnered some experience in the delivery and assessment of a topic of instruction. 

You might even find that you've got the knack to teach. Perhaps students respond to you, or you might have the ability to explain difficult concepts in a way that is easy to understand, or you might simply enjoy the experience. Guest lecturing is the start. You can expand this by co-teaching a subject or module, leading a module yourself, then leading an entire degree program. This is the typical expectation of PhD students, Assistant Lecturers, Lecturers and Senior Lecturers in a number of universities in which I've worked.

If you have the opportunity, I would encourage you to expand your involvement in teaching and administration. Not only will it round out your skills as an academic, it will give you a taste of duties that you will be expected to fulfill should you choose academia as a career.

Good luck to you.