Below, I have copied and pasted the definition of citta given in the glossary of Ajahn Maha Boowa's Spiritual Biography of Acariya Mun.  The definition is surely not as concise as you would like. Be that as it may. I think it is helpful, however, to those readers who wish to understand citta in the context of the talk.   One can also refer to Appendix II of the same biography.

In my view, the first 2 minutes of the video gave a brief introductory definition, namely, that the citta is:

• the mind's essential knowing nature
• that it is pure and simple awareness
• that the true citta only knows, and exhibits no activities or any conditions

I think that is sufficient to follow along with the remainder of the talk, which to my recollection, gave many descriptive definitions which slant towards the activities and nature of the citta.  From a meditator's perspective, I regard these as being most helpful when studying the mind and its various features.

Here is the definition of citta given in the glossary of the Spiritual Biography of Acariya Mun:

citta: The citta is the mind’s essential knowing nature, the fundamental quality of knowing that underlies all sentient existence. When associated with a physical body, it is referred to as “mind” or “heart”. Being corrupted by the defiling influence of fundamental ignorance (avijjã), its currents “flow out” to manifest as feelings (vedanã), memory (saññã), thoughts (sankhãra), and consciousness (viññãna), thus embroiling the citta in a web of self-deception. It is deceived about its own true nature. The true nature of the citta is that it simply “knows”. There is no subject, no object, no duality; it simply knows. The citta does not arise or pass away; it is never born and never dies.

Normally, the “knowing nature” of the citta is timeless, boundless, and radiant, but this true nature is obscured by the defilements (kilesa) within it. Through the power of fundamental ignorance, a focal point of the “knower” is created from which that knowing nature views the world outside. The establishment of that false center creates a “self” from whose perspective consciousness flows out to perceive the duality of the “knower” and the “known”. Thus the citta becomes entangled with things that are born, become ill, grow old, and die, and therefore, deeply involved it in a whole mass of suffering.

In this book the citta is often referred to as the heart; the two are synonymous. The heart forms the core within the body. It is the center, the substance, the primary essence within the body. It is the basic foundation. Conditions that arise from the citta, such as thoughts, arise there. Goodness, evil, happiness, and suffering all come together in the heart.

Samãdhi meditation provides confirmation of the heart’s significance. When the citta gathers all of its outflowing currents into one point, the calm, still state of samãdhi arises. From the meditator’s perspective, that experience is centered in the middle of the chest. The stillness, the brightness, and the awareness of this experience appear to emanate prominently from the region of the heart. The knowing nature of the citta is pronounced right there. Thus, the true seat of consciousness is in the heart; and it is wise, therefore, to avoid thinking of the “mind” as essentially cerebral and located in the head.

There is a strong tendency to think that consciousness results purely from complex interactions within the human brain, and that when the brain dies, consciousness ceases. This mechanistic view is wholly mistaken. While there is evidence that certain parts of the brain can be identified with certain mental functions, that does not mean that the brain produces consciousness. In essence, the brain is a complex processing organ. It receives and processes incoming data impulses that inform about feelings, memory, thoughts, and consciousness, but it does not generate these mental functions; nor does it generate conscious awareness. That is entirely the province of the citta. (for a more detailed discussion see Appendix II, page 461)