There is much disagreement even amongst botanists, but there are similar characteristics!
Most lilies once to belonged to the family Liliaceae. However more recently, many genera have been placed in other botanical families such as the mat-rushes (genus Lomandra) and grasstrees (genus Xanthorrhoea), now in the families Dasypogonaceae and Xanthorrhoeaceae. Nevertheless, they all have some basic features that place those plants into this lily area of classification.
Lilies can be found in most parts of the world with thousands of species and hundreds of genera. So as can be imagined, they come in many guises. In Australia (under the old classification) there were around 60 genera and 300 species, giving us somewhere around 7-8% of the world's lily species.
A typical lily flower, this is the prolific Chamaescilla corymbosa, or Blue Stars. It is a small herb growing from a tuber each winter/spring, and occurring extensively in all southern Australian States.
Blue Stars, Chamaescilla corymbosa
So what is a lily? Firstly they are herbs; i.e. they are not woody with a cambium layer like most shrubs and trees. The leaves are parallel-veined and not interconnected with other veins (reticulated). The flowers have petals and sepals (sepals are the parts that cover the flower in bud), but with lilies, orchids, etc, they are usually petal-like in both color and texture, although they may differ in shape and size.
Blue Stars often carpet the ground where they occur, making a particularly colorful display.
The flowers are made up of three petals and three sepals (although they can appear to be the same (then both known as tepals), however at the flower base, the sepals are the ones on the outside, with the petals on the inner portion next to the (3 or more often 6) stamens. The ovary sits on top of the petals, not below them. So if you have a plant with all these features, then you have a lily or a plant once grouped with them.
A Wurmbea species (collectively known as Early Nancies) that some botanists have now placed in the family Colchicaceae, although some still retain it in Liliaceae. It is slightly smaller than the Blue Stars above, and some species commonly flower with them.
The pretty Fringe Lily (genus Thysanotus), like the Early Nancies above, have by some botanists been placed in the family Anthericaceae, although some still retain them in Liliaceae. Generally a larger plant than those above, with some creeping/climbing and others with a shrubby appearance. However many are spindly and go unnoticed until they appear in flower, then you cannot miss them.
A closer photo of another Thysanotus species. It is interesting to note that these flowers close at night or on overcast days, only to reopen during sunnier periods. The fringe first folds into the petals, then the petals fold lengthwise and close together. The narrower sepals then partially cover them. Most lilies have this habit of closing at night, although these fringe lilies would rank amongst the most complex and delicate to do so.
Autumn Lily, Tricoryne species
Below is another grass lily, this time Caesia viscida (named in 1990) a rare and little known species previously only collected from the (near Esperance) Cape Le Grand National Park. I discovered three new colonies after a bushfire next door to me. It forms quite a large tussock of about 50 cm diameter, and the approximately 40 cm long leaves are very viscus, i.e. sticky, and usually have sand grains attached near their base. The flowers are hidden, originating from near ground level and are mostly concealed by the spreading leaves. So it was quite exciting to discover the identity of this plant, after having walked past them several times and assumed (not a good idea with plants) that it was a sedge, currently without flowers.
Grass Lily, Caesia viscida
Many people go to great lengths to find orchids, but lilies can be just as exciting to hunt down as like orchids, you must be in the right place at the right time to find them. They are I think just as attractive to view and photograph, and with over 300 species within Australia and still probably new ones to discover, they would prove to be a worthy extension of interest, if not a passion in the making.