This is sometimes called a "sandal gap." See here.
Gap between the 1st and 2nd toes (sandal gap).
The above NIH link lists 58 known conditions with this feature+.
Other sources emphasize "halux varus" as a common cause for a larger interphalangeal gap between the first two toes. (Though, note, such a gap is unlikely to appear as in the OP's photo, which is lacking characteristic "bent" toes commonly seen in such deformations of the big toe and surrounding anatomy).
Hallux varus is a deformity of the great toe that is characterized by adduction of the hallux and medial subluxation of the first MTP joint. [Source]1
- See linked source for photos. See here2 for more technical information.
Interesting, radiopaedia suggests that these two terms (sandal gap and hallux varus) are linked.
A sandal gap deformity, also known as hallux varus, is an imaging observation in antenatal ultrasound (typically second trimester) where there is an expanded first interspace, i.e. the gap between the great toe of the foot from the rest of the toes (likened to the gap caused by a sandal).
I think the intent to synonymize these terms is inappropriate based on the other more detailed and peer-reviewed sources I linked/cited. However, I also include this quote due to their usage of the phrase "expanded first interspace" to describe this gap -- However, I cannot find much reputable relevant usage of this phrase in a quick Google or Google Scholar search, so I don't think it's the broad-usage term the OP is looking for.
Other conditions to rule out
FYI: Plantar plate disruption (or injury) is related but not what you're looking for. It's a v-like gap due to injury vs a genetic anatomical characteristic that you're describing.
Also, according to an article in the The Journal of Rheumatology3, "A gap between toes can be an indication of rheumatoid nodulosis, a relatively benign variant of rheumatic disease."
- However, this condition appears between any toe due to rheumatic disease during life, and images from the linked paper rule it out as the more general genetic morphological condition the OP is asking about.
1. Vanore, J.V., Christensen, J.C., Kravitz, S.R., Schuberth, J.M., Thomas, J.L., Weil, L.S., Zlotoff, H.J. and Couture, S.D., 2003. Diagnosis and treatment of first metatarsophalangeal joint disorders. Section 3: Hallux varus. The journal of foot and ankle surgery, 42(3), pp.137-142.
2. Boike, A.M. and Christein, G., 1994. Hallux varus. Hallux Valgus and Forefoot Surgery, pp.307-312.
3. Prati, C., Brion, B.B., Leclerc, G. and Wendling, D., 2014. Spacing of Toes Reveals Rheumatoid Nodulosis. The Journal of Rheumatology, 41(5), pp.973-974.
+ below is an unformatted quick list of the 58 conditions (listed alphabetically) copied in case the cited link dies. One could certainly explore the literature for each of these conditions to determine if some other common phrase/term is additionally used to describe this gap across these conditions.
Acromesomelic dysplasia 4; Acrootoocular syndrome; Al-Raqad syndrome; ALG12-congenital disorder of glycosylation; Arthrogryposis, distal, type 2B2; Arthrogryposis, distal, with impaired proprioception and touch; Atelosteogenesis type II; Atelosteogenesis type III; Autosomal dominant intellectual disability-craniofacial anomalies-cardiac defects syndrome; Cardiac malformation, cleft lip/palate, microcephaly, and digital anomalies; Chromosome 6q24-q25 deletion syndrome; Clark-Baraitser syndrome; Cleft palate-stapes fixation-oligodontia syndrome; CLOVES syndrome; Coffin-Siris syndrome 1; Coffin-Siris syndrome 5; Congenital disorder of glycosylation, type iit; Coxopodopatellar syndrome; Cranioectodermal dysplasia 3; Cutis laxa with severe pulmonary, gastrointestinal and urinary anomalies; Desbuquois dysplasia 1; Developmental delay with autism spectrum disorder and gait instability; Duane-radial ray syndrome; Ectodermal dysplasia with facial dysmorphism and acral, ocular, and brain anomalies; Endocrine-cerebro-osteodysplasia syndrome; Growth delay due to insulin-like growth factor I resistance; Hydrocephalus-costovertebral dysplasia-Sprengel anomaly syndrome; Intellectual developmental disorder with dysmorphic facies and behavioral abnormalities; Intellectual disability, autosomal dominant 1; Intellectual disability-facial dysmorphism syndrome due to SETD5 haploinsufficiency; Kohlschutter-Tonz syndrome-like; Larsen-like syndrome, B3GAT3 type; Lethal acantholytic epidermolysis bullosa; Lethal hemolytic anemia-genital anomalies syndrome; Linear skin defects with multiple congenital anomalies 2; Mandibuloacral dysplasia progeroid syndrome; Meier-Gorlin syndrome 6; Menke-Hennekam syndrome 1; Menke-Hennekam syndrome 2; Microcephalus cardiomyopathy syndrome; Micrognathia-recurrent infections-behavioral abnormalities-mild intellectual disability syndrome; Microphthalmia with limb anomalies; Myofibrillar myopathy 10; Neurodevelopmental disorder with dysmorphic facies and distal limb anomalies; Neurodevelopmental, jaw, eye, and digital syndrome; Nicolaides-Baraitser syndrome; Oculofaciocardiodental syndrome; Orofaciodigital syndrome 18; Orofaciodigital syndrome V; Oto-palato-digital syndrome, type I; Seckel syndrome 1; Short stature, facial dysmorphism, and skeletal anomalies with or without cardiac anomalies 1; Short stature-optic atrophy-Pelger-HuC+t anomaly syndrome; Short-rib thoracic dysplasia 16 with or without polydactyly; Specific granule deficiency 2; Toes, space between first and second; X-linked intellectual disability Cabezas type; Zechi-Ceide syndrome;