There isn’t, as yet, any definition of a 'lifeform' which can withstand logical scrutiny.

Exceptions can easily be found for any definition so far presented. For example (from The American Heritage Dictionary of the English Language):

The property or quality that distinguishes living organisms from dead organisms and inanimate matter, manifested in functions such as metabolism, growth, reproduction, and response to stimuli or adaptation to the environment originating from within the organism."

There are, however, no plausible reasons (apart from our current lack of expertise) why a synthetic system could not be built which could carry out all of the requirements above. But most would argue that such a machine would still not be a ‘lifeform’. (see Synthetic Biology section below)

Many other definitions are circular e.g."Life forms carry out biological processes." or, "A biological process is a process of a living organism." or, weaker still, "The characteristic state or condition of a living organism."

(All definitions via Wikipedia)

A 2016 review in Acta Biotheor. 64(3):277-307. suggested a set of ten measures which, the authors propose, can define whether an entity is alive or not.

It is the regulation of networks and a hierarchy of complicated time patterns, which coordinate the multiple functions on different system levels within the organism." [ which define a lifeform ]

[However]

Despite all modern knowledge about living organisms, it is still difficult to answer the question of what life really is. In a certain sense, life is still an enigma."

See :Properties of Life: Toward a Coherent Understanding of the Organism

Perhaps the strongest (tongue in cheek?) definition so far is from physicist Fritjof Capra "Anything which contains DNA and is not dead."

Bearing in mind all of the above, a comprehensive resumé of the currently accepted best-shot requirements for an entity to be called 'alive' can be read here at Encyclopædia Britannica.

In the 1970s, the concept of 'Autopoiesis' was suggested by Chilean biologists Maturana and Varela. The core idea being that living systems are able to maintain themselves and repair damage via complex feedback loops - and might even be defined by this ability.

There are, however, non-living natural systems which - given a reliable energy supply and a supply of the relevant resources - can self-sustain and even continually 'repair' themselves over long periods (example :Beach cusp formationplugin-autotooltip__plain plugin-autotooltip_bigBeach cusp formation

Beach Cusps are the very well-known shoreline formations which take the form of a repeating, curved 'sawtooth' pattern. They can be composed of various grades of sediment, i.e. sand, pebbles, mud etc..

They can form a large-scale arrays, sometimes stretching for kilometres along a beach. The cusps vary in spacing and size - with regular spacings of a few metres up to 60 or so.).

The concept was later extended to suggest that autopoietic living systems can not only maintain themselves but also improve their state over time. In other words, a new suggestion for evolution. This idea, however, is not universally accepted.

Working from the other direction, many research groups worldwide are attempting to create various forms of 'artificial' life.

So far, building a working replica - or analogue - of even the simplest possible living cell has not proved possible.

A major goal of synthetic biology is to understand the transition between non-living matter and life. The bottom-up development of an artificial cell would provide a minimal system with which to study the border between chemistry and biology. So far, a fully synthetic cell has remained elusive, but chemists are progressing towards this goal by reconstructing cellular subsystems.

See : Synthesis of lipid membranes for artificial cells Nature Reviews Chemistry (2021)

Also see : Life on Earth (timespan)plugin-autotooltip__plain plugin-autotooltip_bigLife on Earth (timespan)

There is currently no scientific agreement as to how long ago life on Earth evolved. Estimates, based on geological evidence, vary by about 200 million years - from between 3.85 billion years ago (Ga), and 3.65 Ga..

To date, the oldest undisputed fossils appear in rocks from 3.2 Ga, but recent carbon-dating techniques applied to rocks from

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