Short answer. Slight physical injuries. Article 266 covers injuries which do not prevent the offended party from engaging in his habitual work and do not require medical assistance. It is the lowest grade of the offence — still a crime, and the grading is settled on evidence of the injury rather than on how minor it seemed.
What the law says
when the offender has caused physical injuries which do not prevent the offended party from engaging in his habitual work nor require medical assistance
Revised Penal Code, Article 266 — Slight Physical Injuries. Read the full provision →
The article has three rungs, and this is the middle one
Article 266 punishes slight physical injuries in three distinct situations. The first is injuries that incapacitate for labour, or require medical attendance, from one day to nine days. The second is the one described in the question: when the offender has caused physical injuries which do not prevent the offended party from engaging in his habitual work nor require medical assistance. The third reaches a person who ill-treats another by deed without causing any injury at all. Each rung carries its own treatment, and the article is precise about the differences between them.
No missed work and no doctor does not mean no offence
This is the assumption most people arrive with, and the article contradicts it plainly. Injuries that leave the person able to work and needing no treatment are still named as a crime, and the third rung goes further still — maltreatment by deed is punishable even where no injury results. What the absence of incapacity and treatment does is fix the grade at the lowest one available, which is a real difference in consequence. It is not a finding that nothing happened, and it does not dispose of the matter by itself.
The grade is proved, not agreed
Which rung a case sits on is decided on evidence, and the two sides frequently disagree about it. A complainant who felt fine on the day may seek treatment later; a defence may point to the person's uninterrupted work. What survives is the record made close to the event: photographs of the injury, the blotter entry, any certificate whether or not treatment followed, and messages sent at the time. The article carries a fine as an alternative in some situations, and any amount should be read from the current amended text rather than from an older printing.
What to do on either side of it
If you are the one accused, treat the charge as a charge even though it is among the lightest in the Code; what you say in the first days shapes the rest. If you were injured, have it documented now, because the number of days of treatment or incapacity is what would move the case to a higher grade, and that number cannot be reconstructed later from memory. Either way, take the documents to a lawyer rather than relying on an assessment of how serious the incident looked at the time.