There are more than 300 different definitions of the sentence, but not a universal one

33. Cases of transition from simple to composite sentences.
There are sentences that display an intermediary syntactic character between the composite sentence and the simple sentence.
From the point of view of syntagmatic structure they are similar to the simple sentence because they have only one predicative live that is explicitly expressed.
From the point of view of syntactic derivation they are similar to complex sentences because they are derived from 2 or more base sentences.
Semi-compound sentences: with homogeneous parts, only one predicative line is expressed explicitly while the second predicative line is fused with the first. Sometimes also termed "contracted sentences".
I swung around, and [I] found her carefully doing her make-up.
Semi-complex sentences: a semi-composite sentence built on the principle of subordination, i.e. with secondary predication (complex subject, complex object, absolute participial construction) + sentences with a dependent appendix + with the double predicate + with verbless clauses + with detached sentence parts. The second predicative line is weakened as it is expressed by a non-finite form of the verb or implicitly.
SP: I saw him come. < I saw that he came.
DA: He is better than you. < He is better than you are.
DP: The sun rose red < When the sun rose it was red.
VC: When a child, he was happy. < He was happy when he was a child.
DSP: The hawk, serene, was circling in the sky. < The hawk, that was serene, was circling in the sky.