This is a surprisingly complex, debated question. Many English speakers may see a haiku as a three line poem that follows a 5-7-5 syllable structure, and that’s basically it. Teachers have drilled that into us as children.
This interpretation of syllable counts is at best an attempt to approximate Japanese conceptions of haikus in their own language, and this is not the definition most professional English-language haiku writers seem to use. For starters, the Japanese concept of “on” that English speakers translate into “syllables” seems a bit more complicated. Haikus in Japan were a genre of poetry, and like all genres, it evolved over time. But when writers write haikus in English, even if they claim to be basing their work on this tradition, they are really doing something entirely different.
I see my own haikus as a cultural US piece of literature rooted in my own American cultural context. How people in the US thought about Japanese haikus has its own history, largely based on US culture and how people in the United States interpreted Japanese society and literature. I started writing a haiku a day, not because of any affinity with Japanese culture but because the poetry style seemed short and easy to write every single day.
I am also very loose in what constitutes a haiku and what topics I could discuss. I almost always followed the structure of a 5-7-5 syllable three-line stanza (except in some cases where we intentionally broke it) because that constraint gave me friction against which I could be creative.
Some in the United States view haikus – or at least the original Japanese concept of the word – as having certain themes or stylistic features: thematically focusing on nature, having at least one piece of seasonal imagery, possessing a “cut word”, for some employing a certain broader distant vantage point, and so on (for example, see https://www.writersdigest.com/write-better-poetry/senryu-poetic-forms). These are fine conventions, but I generally did not follow them. I allowed my haikus to be just about anything, whether that be the act of writing a haiku itself or a frustration I felt that day. In a few cases, I did happen to write about nature, but that was the exception, not the rule.
This reflects one of the fundamental tensions I see on how English-speaking poets conceive of the genre of “haiku” as a whole. Some English haiku writers see themselves as continuing in the “Japanese tradition” (or traditions) of what a haiku is, adapting those practices to a new language and cultural context, maybe refining some of the details on their way. These are more likely to desire a strict syllable count and maybe thematic attachments to nature and other common “traditional” haiku conventions. Others see the haiku writing process as a type of stepping stone for developing their own forms of poetic art.
If this were a spectrum, we would fall much more on the latter side. On a practical level, we often talked about societal themes given how passionate both of us are for those topics, which may, for a traditionalist, fall closer to the Japanese senryu tradition than haikus (although I don’t think we happened to follow that tradition strictly either).
At the same time, many English-speaking poets with a looser sense of what constitutes a haiku follow strict syllable counts – often making their haikus 2 or 4 lines or some other number of lines long – or use haikus for purposes substantially different from what the traditionalists might view haikus. On a practical level, I found the 5-7-5 syllable constraint useful to both channel my thoughts and force me to improve our writing, thus almost always abiding by it.
I think most English poets who try to write haikus too easily think of themselves as continuing a Japanese tradition they are not a part of. Words and ideas travel across cultural contexts, changing in the process. We should accept this. When I, at least, write a haiku, I am doing something fundamentally different from what Matsuo Bashō or Yosa Buson centuries ago in Japan. Yes, we call them “haikus,” a word that happens to come from a similar word for what they used to describe their own work but transformed as it became a part of the English lexicon. What they wrote was fascinating; don’t get me wrong, but what I do when I write haikus is still my own personal reflection situated in my own cultural context.
