There are two places to look for a definition for concealed. The penal code and case law.
California Penal Code
Penal Code 25400 (was 12025) defines the crime of carrying a concealed firearm:
25400. (a) A person is guilty of carrying a concealed firearm when the person does any of the following: (1) Carries concealed within any vehicle that is under the person's control or direction any pistol, revolver, or other firearm capable of being concealed upon the person. (2) Carries concealed upon the person any pistol, revolver, or other firearm capable of being concealed upon the person. (3) Causes to be carried concealed within any vehicle in which the person is an occupant any pistol, revolver, or other firearm capable of being concealed upon the person.
Section (1)(b) does say
(b) Firearms carried openly in belt holsters are not concealed within the meaning of this section.
Penal Code 12001 has some related definitions:
12001. (a) (1) As used in this title, the terms "pistol," "revolver," and "firearm capable of being concealed upon the person" shall apply to and include any device designed to be used as a weapon, from which is expelled a projectile by the force of any explosion, or other form of combustion, and that has a barrel less than 16 inches in length. These terms also include any device that has a barrel 16 inches or more in length which is designed to be interchanged with a barrel less than 16 inches in length. (2) As used in this title, the term "handgun" means any "pistol," "revolver," or "firearm capable of being concealed upon the person."
As a guide to governing one's behavior, taking the 'common understanding' of the term as correct is simplest.
Since the Legislature has not defined concealed, it is strongly recommended to look to what the California Court of Appeals have written over the years through various criminal cases.
Here is Webster's Ninth New Collegiate Dictionary (1983) definition of conceal:
conceal 1: to prevent disclosure or recognition of 2: to place out of sight.
California Case Law
It appears that the California courts have defined concealed as "substantially concealed" but have made that a very narrow so that if a firearm (or dirk/dagger) is partially concealed and even "printing" it's still concealed under 12025.
People v. Fuentes , 64 Cal.App.3d 953 (1976)
Fuentes contends that there was no evidence to show that the dirk was concealed and argues that "[t]here is not even a suggestion in the record that the dirk was ever anywhere but in plain sight." The dirk obviously was not in plain sight. This is not a situation where the weapon was carried openly in a sheath or attached to a belt. fn. 1 The dirk was in Fuentes' waistband.  The mere fact that some portion of the handle may have been visible makes it no less a concealed weapon. A defendant need not be totally successful in concealing a dirk to be guilty of violation of Penal Code section 12020, subdivision (a). (See People v. Hale, 43 Cal.App.3d 353, 356 [117 Cal.Rptr. 697]; People v. May, 33 Cal.App.3d 888, 891 [109 Cal.Rptr. 396]; People v. Tarkington, 273 Cal.App.2d 466, 469 [78 Cal.Rptr. 149].)
People v. Hale , 43 Cal.App.3d 353 (1974)
In our opinion concealment of an essential component of a visible weapon, when done in such a fashion as to make the weapon readily available for use as a firearm, presents a threat to public order comparable to concealment of the entire firearm and falls within the prohibition of section 12025. [1b] In the light of the bizarre arsenal of weaponry Hale had installed around the driver's seat of his automobile, Officer Price had reasonable cause to suspect that a clip and ammunition for the automatic pistol might be hidden close at hand and to make a search for them.
People v. May , 33 Cal.App.3d 888 (1973)
In People v. Tarkington, 273 Cal.App.2d 466 [78 Cal.Rptr. 149], a deputy sheriff during a routine bar check observed what appeared to be an outline of a handgun under appellant's shirt. The deputy felt the object which appeared to be a gun then removed it from the appellant's shirt. It was a loaded .32 pistol. In holding that the weapon was concealed, the court stated at page 469: "We note, also, that appellant's ineptness in accomplishing complete concealment of the weapon did not relieve him from commission of the crime in the presence of the officers. (See People v. Linden, 185 Cal.App.2d 752 [8 Cal.Rptr. 640]; People v. McFarren, 155 Cal.App.2d 383 [317 P.2d 998].)"
People v. Flores (2007) , Cal.App.4th (2007)
As appellant concedes, this contention has been addressed and rejected by reviewing courts. The defendant in People v. Wharton (1992) 5 Cal.App.4th 72 made a similar argument in the context of sufficient evidence. In Wharton, the court determined that "[o]nly substantial concealment is required. [Citation.] 'A defendant need not be totally successful in concealing a dirk to be guilty of violation of ... section 12020, subdivision (a).'" (Id. at p. 75, quoting People v. Fuentes (1976) 64 Cal.App.3d 953, 955.) "The mere fact that some portion of the handle [of the defendant's dirk] may have been visible makes it no less a concealed weapon." (People v. Fuentes, supra, at p. 955.) We agree with the reasoning of these cases and conclude that total concealment is not required. Thus, the jury was properly instructed with CALCRIM No. 2501.
People v. Wharton (1992) 5 Cal.App.4th 72 , 6 Cal.Rptr.2d 673 (1992)
Defendant contends there is insufficient evidence of concealment because the tip of the knife was protruding from his pocket. We disagree. Only substantial concealment is required. (People v. Fuentes (1976) 64 Cal.App. 3d 953, 955 [134 Cal.Rptr. 885] [dirk with handle protruding from waistband found to be a concealed weapon].) "A defendant need not be totally successful in concealing a dirk to be guilty of violation of Penal Code section 12020, subdivision (a)." (People v. Fuentes, supra, 64 Cal.App.3d at p. 955.) Here, the jury was permitted to examine the knife, which our perusal discloses is approximately seven and three-eighths inches long. The jury was apprised that only one and one-half to two inches of the blade were protruding from defendant's pocket. These facts support a finding of substantial concealment.