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.”
But there are no plausible reasons (apart from our current lack of expertise) why a robotic system could not be built which could carry out all of the requirements above. And many would argue that such a machine would still not be a ‘lifeform’.
Many other definitions are circular e.g.
“Life forms carry out biological processes.”
“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)
Perhaps the strongest 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 requirements for an entity to be called 'alive' can be read here at Encyclopædia Britannica.