Back to the OP question: what is enlightenment?
My response to this is based on what Shakymuni Buddha said about his teaching: "I teach one thing and one thing only: an end to suffering."
So if you want to know if you (or anyone else) is enlightened, this is the litmus test. Are you suffering? Are they suffering? Seems pretty simple to me.
"An end to suffering" has always been the "goal" of Buddhism....
What is enlightenment? Enlightenment is the end of suffering.
That is liberation. Enlightenment, anutarasamyaksambodhi, is the complete and total extinction or purification of negativities and obscurations in any form AND the complete and total development of all good qualities. This definition comes from the mouth of several Tibetan Buddhist lamas but this is a sutric
definition that also carries over into tantra. This is in fact the etymology of the word Buddha in Tibetan, sangye.
From Khenpo Karthar Rinpoche
This is the meaning of SANGYE, which is the primary word in Tibetan for a Buddha. SANG means "elimination" and GYE means "development." What did he eliminate? The meaning of SANG is the absence of any stain at all. "Elimination" means there is absolutely no stain, obscuration, or fault. Because of this stainlessness--the immaculate nature of the enlightened being--there has never been a case in which the Buddha was unable to fulfill someone's wish or answer someone's question. The absence of these shortcomings shows the complete removal of obscuration and stain. Therefore, SANG also means the absence of such inabilities in the enlightened state of the Buddha. If someone has a weakness, he or she is not SANG, because all faults are not removed.
GYE is translated as "developing." This means that all the enlightened qualities are fully developed. Because of the power and strength of the full development of enlightenment, Buddha was able to understand the needs of each sentient being. This omniscience is the quality of full development. Not only did he understand the need, he was able to provide whatever remedy would fulfill the wish of every sentient being.
When we understand these enlightened qualities, we do not use the term Buddha or SANGYE lightly. We learn to respect what it means when we understand all of these qualities. Otherwise, we often fail to recognize the full meaning of SANGYE, and once we gain a little experience we think, "I am enlightened." When we develop a little power or strength, we think, "I am enlightened." We must understand the full meaning of enlightenment, and that there is no weakness in an individual who is fully enlightened, or SANGYE.
However short of anutarasamyasambodhi, full Buddhahood, there are several levels and stages to enlightenment. Generally people who have attained the 1st Bhumi, the Path of Seeing, could be said to be enlightened but they are still far from complete enlightenment.
So what about Arhats? They have no more afflictions and negativities but are still obscured in knowledge and omniscience (furthermore, Arhats display a lack of omniscience at least in the Mahayana Sutras and specifically ask Shakyamuni Buddha for specific teaching. Some people encountering this in Mahayana are upset by this but it should be emphasized that Arhats are also part of the Aryan (Noble) Refuge Field [Arhats are included as objects of refuge in the Mahayana]). What about Pretyakabuddhas? They still haven't attained full enlightenment and are sort of 3/4 Buddhas but lack full omniscience and full development of positive qualities like compassion (but they are said to display miracles as part of their mostly silent teaching).
_________________Kirt's Tibetan Translation Notes
"Only you can make your mind beautiful."
HH Chetsang Rinpoche