Sanskrit, literally "ford or place of crossing." A tirtha originally referred to somewhere where one could cross a river- no more, no less. However, over the years, it has taken on a quite a range of meanings in Hindu thought, and now is often used generically to refer to a holy site or place of pilgrimage. However, a little examination of the literal definition provides a glimpse at some quite exciting Hindu thought regarding divinity and transcendence.
Relating a crossing point to a holy site is not particularly difficult. There's hardly a river in India that does not have some association with the story of one of the many gods or avatars that occur in Hindu myth. Other waterways are associated with local spirits or lesser gods- a naga, perhaps. And, of course, the Ganges is the mother of all rivers, and its sanctity feeds into every drop of water on the sub-continent. So, with the divinity of rivers assured, places to bathe and wade in them- thus cleansing oneself with the sacred water- quickly became themselves holy sites. Nowhere is this more clear than on the ghats of the city of Kashi (called Banaras in the West), where pilgrims flock literally by the millions to bathe in the water of the Ganges. In narrower rivers than the Ganga, spots where it was safe to bathe were often also spots where the water was calm enough, and the bottom stable enough, to ford the river safely. So a place of crossing, a place of bathing, and a sacred river are all united in the concept of the tirtha, the crossing.
Soon, this concept became a bit more abstract. The avatar of a god, for instance, is a being that crosses the line between humanity and divinity. The avatara itself is a tirtha, a bridge that spans the gap between heaven and earth. Humans can be concretely united with the gods through the presence of an avatar; by knowing Krisna or Ram, we know Vishnu.
In the modern day, when the streets don't seem to be overflowing with the mighty avatars of benevolent gods, the tirtha becomes again more abstract- residing in a structure such as a temple, or in the image of a god. When a devout Hindu enters the temple of their god and offers puja before the image of the god, they know quite well that the lump of rock or metal before them is not the god itself. Rather, the god has been invited to reside in the image, so that the devotees might better themselves by revering it. The god-image to which offerings are made, and even the temple itself can be said to be a tirtha, a crossing that permits the veil between man and the gods to be drawn briefly back to allow the darsan (seeing or viewing) of the divine. The image and the temple provide the human mind with a way of interacting with a divinity that is itself beyond comprehension, thus bridging our understanding and the understanding of the gods.
Yet more abstractly, a tirtha can be a way of thought or a religious practice. Almost every Hindu (and many Buddhist) sacred writing mentions at one point or another the image of crossing the river- that is, leaving the world of samsara for the release of moksha. The methods of discipline that lead to this release - the various yoga- are themselves tirtha, for they allow one to cross to the farther shore of liberation. In much the same fashion, the guru or teacher who teaches us these methods of liberation is him or herself a tirtha, for they also are a means of crossing to the other shore. Here we see an important continuity between Hindu and Buddhist teachings- the Buddha and his Dharma can be seen as a tirtha, and the analogy fits cleanly with the language used throughout the Pali Canon and other sources.
How about an entire city? Kashi, the city where Shiva resides when he destroys the world to bring about its rebirth, is said to be the one place where rebirth out of samsara is assured- because it is the one spot in the universe that is never destroyed, or because Shiva himself whispers the mantra that gives liberation into the ears of the dying. Thus, the entire city of Banaras is one enormous tirtha, providing a crossing that ferries the faithful (or, for that matter, anyone who dies there) across to the other shore.
Thus, we can see that a tirtha is much more than a holy site, and much more than just a river crossing. It is at once both- it is a holy place that can be a person, place, thing, or idea, and yet which is always a form of crossing. The tirtha crosses rivers, worlds, and realms of existence, granting liberation or an encounter with the divine.
A number of lectures by Diana Eck
given at Harvard
for various courses in the study of religion. Her books "Encountering God", and "Banaras: City of Light" cover the concept, and a number of others, in some detail.