In poetry a form which consists of, initially, four lines, the first having 1 syllable, the second 2, the third 3 the fourth 4. These are followed by a fifth coda line containing 10 syllables (the sum of lines 1-4). the whole is centred to form a triangle
Also quite common is the double tetractys where the first tetractys is followed by a second reversed, so that the whole forms a diamond shape.
An example of the latter - my first and so far only attempt in the form - is below
stars
planets
asteroids
and nebulae
Litter this galaxy, this Milky Way
The sky's so crowded, I wonder – don't you -
in cosmic terms
Who will care
what I
do?