Half note is the second longest base form used in music notation. It duration is half of the whole notes duration.
Whole note is the longest note used in music notation and rhyth,. It’s duration can be altered by a dot or a tie but in its base form it is still the longest.
Rhythm is present everywhere in the Universe – the circle of the Sun, Erths orbital around the Sun, the turning of the World. And from there the circadian rhythm which is present on the level of genes of every lifeform on the planet Earth.
Half step is the smallest possible distance between two notes on the piano keyboard. So it is like the tiniest building brick of all music on the keyboard.