3"1/2'' The Tibetan word is Chorten, which means "the basis of offering".
It is a symbol of enlightened mind, (the awakened mind, universal divinity) and the path to its realization.
If you had to use just two words, the best definition I have seen is "Spiritual Monument"
The stupa represents the Buddha's body, his speech and his mind, but most especially his mind and every part shows the path to Enlightenment
"The visual impact of the stupa on the observer brings a direct experience of inherent wakefulness and dignity. Stupas continue to be built because of their ability to liberate one simply upon seeing their structure" - Chogyam Trungpa Rinpoche
Every stupa contains at the very least a life tree and holy relics:
"When a great teacher passes away, his body is no more, but to indicate that his mind is dwelling forever in an unchanging way in the dharma kaya, one will erect a stupa as a symbol of the mind of the buddha''